"First conditional" or "conditional I" refers to a pattern used in predictive conditional sentences, i.e. those that concern consequences of a possible future event. In the basic first conditional pattern, the condition is expressed using the present tense (having future meaning in this context), and the consequence using the future construction with will (or shall):
If you make a mistake, someone will let you know.
If he asks me, I will/shall consider his proposal carefully.
Here are some more examples:
If it rains, I won't go to the park.
If I study today, I'll go to the party tonight.
If I have enough money, I'll buy some new shoes.
She'll be late if the train is delayed.
She'll miss the bus if she doesn't leave soon.
If I see her, I'll tell her.
The use of present tense in dependent clauses with future time reference is not confined to condition clauses; it also occurs in various temporal and relative clauses (as soon as he arrives; take the first train that comes; etc.).
The present tense used in the condition clause may take the form of the simple present as in the above examples, or the present progressive, present perfect or present perfect progressive as appropriate (according to general principles for uses of English verb forms):
If he is sleeping when we arrive, we shan't wake him. (present progressive)
Will you wake him if he hasn't stirred by 10 o'clock? (present perfect)
If you have been working for more than ten hours when he returns, he will take your place. (present perfect progressive)
Although the consequence in first conditional sentences is usually expressed using the will (or shall) future (usually the simple future, though future progressive, future perfect and future perfect progressive are used as appropriate), other variations are also possible – it may take the form of an imperative, it may use another modal verb that can have future meaning, or it may be expressed as a deduction about present or past time (consequent on a possible future event):
If it rains this afternoon, come round to my place! (imperative)
If it rains this afternoon, we can/could/should/may/might find somewhere to shelter. (other modals)
If it rains this afternoon, then yesterday's weather forecast was wrong. (deduction about the past)
If it rains this afternoon, your garden party is doomed. (deduction placed in the present)