Must, have to, should (obligation)

obligation / necessity: have to / must (infinitive)
Must and have to have a very similar meaning.
Have to is more common for general, external obligations, for example rules and laws.
Must is more common for specific or personal obligations.
Compare: I have to wear a shirt and tie at work (It's the rule in this company). I must buy a new shirt - this one is too old now (It's my own decision).
Have to is a normal verb and it exists in all tenses.
Must is a modal verb. The only forms are must and mustn't.
You can also use have to or must for strong recommendations. You have to/must see that film. It's fantastic.

no obligation / necessity: don't have to. ---- prohibition: mustn't (+ infinitive)
Don't have to and mustn't are completely different.
Compare: You mustn't drive along this street (It's prohibited, against the law). You don't have to drive - We can get a train (You can drive if you want to but it's not necessary/obligatory).
You can often use can't or not allowed to instead of mustn't.

advice or opinion: should/shouldn't (+infinitive)
Should is not as strong as must/have to. We use it to say if we think something is the right or wrong thing to do.
Should is a modal verb. The only form are should or shouldn't.
You can also use ought to and ought not to instead of should/shouldn't.

Comparatives and superlatives

Comparing two things (or actions)
Regular comparatives adjectives/adverbs:
hard/harder, big/bigger, easy/easier, modern/more modern,
Irregular comparative adjectives/adverbs:
good-well/better, bad-badly/worse, far/furhter
After than or as we can use an object pronoun me, him, her, etc. or a subject pronoun (I, he, she) + auxiliary verb: She's taller than me or She's taller than I am.

Superlatives
Form superlatives like comparatives but use -est or most/least(lo más mínimo).
You normally use the before superlative, but you can also use possessive adjectives: Their most famous song.

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Present perfect continuous

Present continuous for unfinished actions
Have/Has been + Verb+ing
Use the present perfect continuous with for and since with action verbs.
How long have you been learning Spanish?
He's been studying here since September.
They've been going out together for four years.

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With non-action verbs (know, be, etc.) use the present perfect simple, NOT the present perfect continuous. with for and since. I've known her for ages. NOT I've been knowing her for ages.
With live and work you can use the present perfect simple or continuous with for and since. I've been living here for six months. I've lived here for six months.

Present perfect continuous for recent continuous actions
Use the present perfect continuous for actions which have been going on very recently. They have usually just stopped.
Your eyes are red. Have you been crying? No, I've been cutting onion.

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Present perfect and past simple

Present perfect: have/has + past participle (studied, been, ...)
Past experiences.
I've been to London, but I haven't been to Paris.
She's never met his ex-wife.
Have you ever lost your mobile?
We often use ever and never when we ask or talk about past experiences. They go BEFORE the main verb.
Recent past actions.
He's just arrived at the room.
With yet and already (for emphasis)
I've already done my homework. 
Have you finished yet?
My brother hasn't found a new job yet.
Just and already go BEFORE the main verb in positive sentences, yet goes AT THE END of the phrase in negative sentences and questions.
Unfinished states (non-action verbs) which start in the past and are still true now.
They've known each other for 10 years.
How long have they been married? Since 2003.
Use How long...? + present perfect to ask about an unfinished period of time (from the past until now).
Use for+a period of time, or since with a point of time.

Present perfect or past simple?
Use the present perfect when there is a connection between the past and the present.
Use the past simple to ask or talk about finished actions in the past, when the time is mentioned or understood. We often use a past time expression.
I've been to Barcelona twice.(= in my life up to now) I went there in 1997 and 2009. (= on two specific occasions)
How long have you been married to Jane? (= you are married to Jane now) How long were you married to Jane? (= you are not married to Jane now)
I've bought a new computer. (= I don't say exactly when) I bought it on Friday. (= I say when)

Future forms

Be going to + infinitive
Use going to, NOT will/won't, when you have already decided to do something.
Future plans and intentions
My brother's going to adopt a child.
I'm not going to go to Barcelona next week.
Predictions
It's going to rain.
With the verb go you can leave out the infinitive. I'm going (to go) to New York.

Present continuous: be + verb + ing
You can usually use present continuous or going to for future plans/arragements.
Going to shows that you have made a decision. We're going to get married in the summer.
Present continuous emphasizes that you have made the arrangements. We're getting married on July 12th.
Future arrangements
They're meeting at 10:00.
She's leaving on Friday.

Will/shall + infinitive
Use will/won't for:
Instant decisions. I'll have the steak.
Promises. I won't tell anybody where you are.
Offers. I'll carry that bag for you. Shall I help you with your homework.
Suggestions. Shall we eat out tonight?
Use shall with I and We for offers and suggestions when they are in questions.
Use will or going to for predictions.