In your first four listed examples above, Ruben, you are referring
to future time, but only 2 and 3 are grammatically correct.
look at the arrangement of structures in your second sentence.
behaves like if in subordinate clauses of this kind and remains
in the present tense when we are referring to a future condition.
Consider the following:
'What will happen when the contract expires?'
'What will happen if I get home late?'
'What will you do if I don’t return till Thursday?'
look now at the arrangement of structures from your third sentence.
If we are discussing a general truth, rather than a specific instance
or event, the main clause usually remains in the present tense as
well as the tense in the if or when clause. Consider
'If the roads are icy, I stay at home.'
'What sort of clothes do you wear when the temperature
falls below -15 C?'
'What happens in this company when male staff
request paternity leave?'
in your example, Ruben, we can use the present tense in both clauses
to refer to a future event. It is clear that you are talking about
your specific contract because you refer to it as ‘the contract’.
(If you had said: ‘What happens when contracts expire?’ you would
be discussing general truths.)
your fourth sentence, remember:
does/do/did when what/who question refers to the subject.
'wh-' questions, like what, who or which, refer
to the subject of the sentence, we do not use the auxiliary forms
do/does/did. These auxiliaries are used only when the wh-question
refers to the object. Compare the following:
'Which animals rear
their young in nests?' 'Squirrels rear their young in nests called
'Which animals did you see on safari?' 'We saw lions,
leopards and tigers.'
'Who saw the accident?' 'I saw it.'
'What did you see?' 'I saw the car collide with a bus.'
The exception to this rule is if you wish to use do/does/did for
emphasis when what/who/etc questions refer to the subject.
Thus, normally, we expect to hear:
more than a hundred grand a year in this company?' 'The Chairman
'What happens when the contract expires?' 'Under normal
circumstances, it’s renewed.'
But if the answer is not known and
we wish to emphasise the question in a particular way, use of the
auxiliary would be appropriate. Thus:
'Well, what does happen when your contract expires?
Does anyone know?'
'Well, who does earn more than a hundred grand in this
company? Does anyone?'
fifth sentence happen can be used with a following infinitive
to suggest that something may happen by chance. Consider the following:
'If you happen to see my father, ask him to ring me
at home later this evening.'
'If you happen to need any help with the decorating,
don’t hesitate to ask me.'
'In your example, the context for your statement
might be the following:
'I don’t happen to be a teacher, though I sometimes
give lessons to help out.