What does a Spanish sentence look like? 
This is a vast simplification but I will attempt to show you the principal difference between a Spanish and an English sentence from a beginners point of view. First let's look at how we form an English sentence. If you have not learned another latin language the principal thing that you will find difficult in Spanish is  that the object can come before the verb. See tables.
 

The first thing that you will notice is that it appears that there is no subject. This is because the the subject is contained in the verb in Spanish ("amo = I love") and the subject pronouns are only used when you really want to emphasize the subject. For example if you want to say "I am English" you would say "Soy inglés" You wouldn't normally say "Yo soy inglés"

When I first started learning Spanish one of the most difficult things to get used to  was that the object can come before the verb. As you can see in the example in the table The word order for "I love you"  would be "you I love" The sooner you get used to this the better.

Generally speaking the word order of an English sentence is more ridgid than a Spanish sentence.
 

What do Spanish verbs look like? 

There are three types of verb in Spanish. -AR, -IR and -ER. There is no difference in meaning between them.
Spanish has  an inflexional verb system which means that each verb has a root and an inflexion. The root tells you which verb you are using and the inflection tells you the tense (time)  and which person you are refering to. The sooner you learn the most common inflections the better.

If you want to see a full list of regular Spanish verbs click here and if you want the full conjugation of any Spanish verb click here

How do I ask a question in Spanish? 
In English we change the order of the subject and person in order to make a question.
eg. (affirmative) = They have eaten (interogative) = Have they eaten?
In Spanish there is no change in the word order.  You know that someone is asking a question because there is a change in intonation at the end of the sentence.  In written Spanish there are question marks both at the beginning and at the end of the question. eg  ¿Tienes fuego? (Do you have a light?)

There is also another way to make a question. In English it is called a tag question. eg. You live in Spain, don't you?
This is a difficult system for foreigners learning English but in Spanish it is very easy. You just add "verdad" or "no"
to the end of the question and use a question intonation. For example:
¿Vives en España, verdad? or  ¿Vives en España, no?
Note ("verdad" means "True")
If you speak  Spanish a lot you may find yourself using ¿no? when you go back to English.

How do I make negative sentences in Spanish? 
This is very easy you just put "no" before the verb.
eg. No vivo en Paris.  = I don't  live in Paris.

How do I use adjectives in Spanish? 
As in most latin languages the the adjective goes after the noun. In English it is the opposite.
Eg. In English we say: "A red book" but in Spanish we say "A book  red"
Eg. A red book = Un libro rojo.

All the nouns in Spanish are either masculine or feminine and when we use an adjective they must agree with the gender of the noun. Also the adjective must agree in number so if the noun is plural so is the adjective.
Generally speaking masculine adjectives end in "o" and feminine adjectives end in "a". There are also a lot of adjectives that do not have an "a" or "o" ending eg. verde= green in which case there is no difference when you use them with masculine or feminine nouns.

What are the main difficulties in learning Spanish for an Engish speaker? 
1. In Spanish there are two verbs for "to be". "Ser" and "Estar" You can read the explanations in a grammar book and understand them very well but it will be a long time before you will stop making errors when you speak. The usual explanation is that "ser" is used for permanent things and "estar" is used for temporary states. This is just a brief overview so I won't go into details but just to give you an example look at these two sentences in English. "I am boring" "I am bored" The first means that you are a boring person the second means that you are temporarilly bored by the situation you are in. These two sentences translated to Spanish would be "Soy aburrido" and Estoy aburrido" respectively. For possesions you use "ser" eg "es mio" (it's mine) and for position you use "estar" eg "¿donde está?" (where is it?) 

We now have a full tutorial for "ser" and "estar" on this page

2. The second difficulty is that the subjunctive is used a lot in Spanish but hardly ever in English. For example one of the most common use of the subjunctive in Spanish is with "want". An easy structure for a foreigner learning English could be.
"I want you to buy me a present" In Spanish you would have to say (literally) I want that you buy me a present. or "Quiero que me compres un regalo" -  "You buy"  would normally be "compras" but because you have to use the subjunctive you have to change it to "compres"  which is the subjunctive form.   Although the subjunctive is difficult, if you don't use it properly you will probably be understood so it's not such a big problem.
We now have a page about the Spanish subjunctive on this page

3. All nouns in Spanish are either masculine or feminine.
Eg. El Libro = the book (masculine)
Eg. La casa = the house  (feminine)

Generally speaking masculine nouns en in "o" and feminine nouns end in "a"
There is no particular communicative advantage in having masculine and feminine nouns. I don't understand why all the latin languages evolved with such a seemingly pointless complication so you will have to learn a lot of extra infromation for no particular communicative advantage.

By the way some of them are not regular: (here are some very common ones) la mano = hand - la foto= photo - el día = day - el mapa = map - el programa = program - el sistema = system - el problema = problem -el clima = climate

4. Spanish has an informal and a formal form of address.
EG. What is your name? is:-
Formal: ¿Como se llama usted?
Informal: ¿Como te llamas?
(literally. - What do you call yourself?).
It is very difficult to know which one to use and there are different social norms especially in South America. The formal form is becoming less used. If you ask a Spanish person to explain it to you each person will give you a different answer. I personally only use "usted" if I am speaking to someone's grandparents but my neighbours who have lived next door to each other for 40 years call each other usted. By the way did you know that a formal and an informal form of address used to exist in English?  (hence the use of "thou", "thee", "thy", "thine", and "ye") The informal form of you was dropped because for over 100 years it was forbidden on pain of death in England, it must have been at the time of the Puritans.  The word "you" comes from "thou" which was the formal form.
Aparently what I say may be wrong. I have receive several e-mails on this subject. Click here if you want to know more

5. The big problem with learning any language is understanding what people say when they are speaking fast.

 What are the easiest things about  learning Spanish for an Engish speaker? 

1.In English there are 13 vowel sounds in Spanish there are only five (aeiou) Spanish is a syllable timed language so the vowels stay the same length no matter where they are in a sentence. Imagine how difficult it is for a learner of English when most English words have a long and a short form or a strong and a weak form. Look at the sentence: Do you think John would like to go for a drink? In English this could be pronounced "De yu think John ud like tu go fe u drink?" This problem normaly doesn't exist in Spanish because all the vowels are fully pronounced. Sometimes in regional accents some of the consonants are dropped for example here in Andalucía they drop the final "s" from most words.  The is also a  widespread silent consonant in  the past participles eg. "hablado" = spoken.  Sometimes the Spanish do not pronounce the "ado" so "hablado" would be said as "hablao" However Spanish pronunciation is quite easy to learn although it is difficult to speak without a foreign accent. Maybe, good advice would be to learn the vowel sounds and then pronounce everything. If you have Spanish friends who speak a little English you may notice that they try tend to over-pronounce.
By the way. The Spanish word which contains all the vowel sounds is "murciélago" which means bat (the one that flies not the baseball type).

We now have a Spanish pronunciation guide with all Spanish sounds click here

2. It is very easy to write in Spanish. Once you have learned which sound corresponds to each letter you can easilly write correctly in Spanish. The only difficult thing with writing is the accents.

3. It is easy to make afirmative, negatives and  questions.

4. Just open any dictionary and you will find that 50% of the words are very similar.