Realis conditions

The verb in the condition clause is in the past tense (with a past tense interpretation) or in the present tense (with a present or future tense interpretation). The result clause can be in the past, present, or future. Generally, conditional sentences of this group are in two groups, the zero conditional and the first conditional. This class includes universal statements (both clauses in the present, or both clauses in the past) and predictions.

The zero conditional is formed with both clauses in the present tense. This construction is similar across many languages. It is used to express a certainty, a universal statement, a law of science, etc.:

If you heat water to 100 degrees celsius, it boils.
If you don't eat for a long time, you become hungry.
If the sea is stormy, the waves are high.

The first conditional is used more generally to express a hypothetical condition that is potentially true, but not yet verified. The conditional clause is in the present or past tense and refers to a state or event in the past. The result can be in the past, present, or future. Some examples with the condition clause in the past tense:

If she took that flight yesterday, she arrived at 10pm.
If she took that flight yesterday, she is somewhere in town today.
If she took that flight yesterday, we'll see her tomorrow.

A condition clause (protasis) in the present tense refers to a current state or event (which may or may not be true), a state or event that could be verified in the future, or a future event. The result can be in the past, present, or future:

If it's raining here now, then it was raining on the West Coast this morning.
If it's raining now, then your laundry is getting wet.
If it's raining now, there will be mushrooms to pick next week.

If it rains this afternoon, then yesterday's weather forecast was wrong.
If it rains this afternoon, your garden party is doomed.
If it rains this afternoon, everybody will stay home.

If I become President, I'll lower taxes.

Future tense forms are not used in the condition clause (protasis) in English: *If it will rain this afternoon, …