Overview
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.