Tenses and auxiliary verbs

Here, I wish to throw some light on tenses used in English, common mistakes that we make and confusion existing between diff auxiliary verbs...

Auxiliary verb: A verb that combines with another verb in a verb phrase to help form tense, mood, voice, or condition of the verb it combines with

A commom mistakes (in red):

Q: Ma'am, can i come in?

A:(from ma'am) You can, but you may not!!

It should be 'may i come in?' When you use 'can', you test your own ability to come in. As a reason Ma'am says 'you can', that means you have the ability to come in, 'but you may not', means she dosen't want you to come in...

Jose should study tonight => it is Jose's responsibility to study tonight

Jose can/could study => he has the ability to study

Jose would study => he intents to study

Jose may/might study tonight => there is a possibility that Jose studies tonight

I was to have watched the movie => telling abt past

I was to watch the movie => I am supposed to be inside the thethre watching the movie, but got late..ehh

I am to watch the movie => I am looking forward to watch the movie

Auxiliary verb used with a habit: 'used to'

eg: He is a spoilt child, and is used to disobeying his elders.

Had had:

I would have gone on a trip to America if I had had enough money.

had: describes possession

had: to assert it happened in past