Hello
[1] He should told me yesterday. 
=> He should have told me yesterday. 
See Learning English | BBC World Service
[2]
She could play the piano. <could = past>
=> COULD + bare infinitive
She can play the piano. <can = present>
Note that, bare infinitives lack tense, which is why could and can carry tense. There's this rule: only one verb per simple sentence can carry tense.
[3]
- You could do this right now (, if you wanted to). <conditional>
- You can do this right now. <permission>
- She can <present> type well, and she could <past> type well as/(when she was) a trainee.
[4] Would (a modal, a modal auxiliary verb, modal auxiliary) is a type of auxiliary verb.
If it
is sunny tomorrow, I
might go to the beach.
(Going to the beach is only a possibility)
If you
come early, you
can see my mother before she leaves.
(It is possible for you to see my mother)
http://www.eslbase.com/grammar/first-conditional
When we use
will in
the result clause of the first conditional, we are certain that something will happen. It is
definite. But we can use
may or
might instead of
will. This means that the consequence is not certain.
It is possible, but not definite.
I'm not feeling very well. If it rains tomorrow, I’
ll stay at home.
I'm feeling fine and I've got a car. If it rains tomorrow, I
might stay at home, or I
may go shopping. I'll decide tomorrow.
Note: In the first conditional,
there is no difference in meaning between
may and
might.
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