Some general guidelines for verbs! Guia general para verbos.

VERBS:  There are two kinds of verbs in English: special verbs & normal verbs. There are 13 special verbs, all the rest are normal verbs.
NORMAL VERBS
They can only make affirmative sentences. For all the other things, they need DO:
affirmative   I live in London
negative   I don't live in London
interrogative   Do you live in London? Yes, I do.

Normal verbs can be regular or irregular!

SPECIAL VERBS
They never use DO, they can do everything. Most of them can use contractions.
affirmative   I can speak English
negative   I can't speak English
interrogative   Can you speak English? Yes, I can.

The special verbs are: to be, have, do, can/could, will/would, shall/should, may/might, must, ought to
There are two kinds of special verbs: auxiliaries and modals.
AUXILIARY VERBS (or HELPING VERBS)
The auxiliaries verbs in English are: to be, have, do, will, would.

They help a normal verb to make some tenses. A normal verb only has present and past tense, but we can make more tenses using auxiliary verbs:

present: I live in London
past: I lived in London
present perfect : I have lived in London   auxiliary: Have.
future: I will live in London   auxiliary: Will.
conditional: I would live in London   auxiliary: Would
present continuous: I am living in London   auxiliary: Be.
passive voice: I am called Tim   auxiliary: Be.
present perfect continuous: I have been living in London   auxiliaries: Have & Be.
negative: I don't live in London   auxiliary: Do.
etc.

To Be   to make continuous forms and the passive voice: I am writing a letter  -  The window was broken
Have   to make the present perfect: He has lived here for 5 years
Do   to make negatives, interrogatives, etc: I don't like tennis / do you know him? Yes, I do.
Will   to express future: They will come to visit me
Would   to make conditionals: If you had problems, I would help you.

MODAL VERBS
Modal verbs are used to express ideas such as possibility, intention, deduction, suggestion, obligation, etc. 
Modal verbs never change, they don't add an -S for the 3rd person singular and they don't use ¨TO¨ before or after them (exceptions: to be, ought to):
normal: I live here, he lives here
modal: I can swim, he can swim
normal: I want to go
modal: I may go
POSSIBILITY: May & Might
- It may rain today because it is very cloudy.
- If you go to Hollywood you might see some famous actors in the street.
PERMISSION: Can, May
- Can I go to the toilet?
- May I go to the toilet? (more polite)
ABILITY: Can (past tense and conditional tense: Could)
- I can play the guitar
- When I was a child I could play the guitar, but now I can't.
ADVICE: Should
- If you want to be a doctor you should study more.
INTENTION: Will (past: Would)  [yes, this verb can be a modal or an auxiliary for the future)
- Don't worry, I will help you.
- He said that he would help me.
INVITATION: Will
- Will you come to my party tonight?
- Sure, I will.
DEDUCTION:  Must ("can't" for the negative)
- Who's that? - That must be John, because he is very tall.
- No, that can't be John, John is younger.
SUGGESTION: Shall
- Shall we go to the cinema?
OBLIGATION: Must
- You must stay here and you mustn't speak to anyone until I come back home
PROHIBITION: Can (in the negative)
- I'm sorry, you can't park here, there is a yellow line on the road.

SUMMARY
The 3 No's: Remember this simple rule for the use of special verbs (modals & auxiliaries): …NO -¨S¨, NO ¨TO¨, NO ¨DO¨.

NO –S   I can/ she can
NO TO   I can walk / no infinitive
NO DO   you can / you can’t / can you?