Saylor.org's English Composition/Conditionals

In English, there are three contexts for conditional sentences.

The first context involves the possible outcome of an event that is likely to occur:

If + Present Simple/Present Progressive + Present Simple/Present Progressive/Future Simple/Future Progressive/Imperative

• If + I miss the bus, + I will be late for school

The second context involves the possible outcome of an event that is less likely to occur; various sentence structures are possible for this context:

If + Present Perfect/Present Perfect Progressive/Preterite/Past Continuous + Full Infinitive + Conditional Present/Conditional Present Progressive

• If + I was + to miss the bus, + I would be late for school.

In some dialects: If + Imperfect Subjunctive + Full Infinitive + Conditional Present/Conditional Present Progressive

• If + I were + to miss the bus, + I would be late for school.

In some dialects: If + modal auxiliary "should" + Bare Infinitive + Conditional Present/Conditional Present Progressive

• If + I should + miss the bus, + I would be late for school.

The latter two formulations, with the imperfect subjunctive or the modal construction, can be modified to use subject-verb inversion instead of the conjunction if:

• Were I + to miss the bus, + I would be late for school.
• Should + I miss the bus, + I would be late for school.

The third context involves the hypothetical outcome of an event that did not occur:

If + Pluperfect/Pluperfect Progressive + Conditional/Conditional Progressive/Conditional Perfect/Conditional Perfect Progressive

• If + I had missed the bus, + I would have been late for school.

Here too the conjunction if can be replaced by subject-verb inversion:

• Had I missed the bus, + I would have been late for school.

In each formulation it is possible to reverse the order of the clauses; however, the protasis must always follow the word "if" or exhibit subject-verb inversion:

• If + I miss the bus, + I will be late for school can be adjusted to I will be late for school + if + I miss the bus.
• Should + I miss the bus, + I will be late for school can be adjusted to I will be late for school + should + I miss the bus.