Do I use the bare infinitive or "to" after "was" in do-clefts?

Both are accepted here: "What he did was leave" and "What he did was to leave". Keep the verb in its base form in either pattern.

When should I use an event cleft instead of a do-cleft?

Use a do-cleft when a person or group performs the action. Use an event cleft when you frame the whole occurrence: "What happened was that the bus never arrived."

Home Grammar Cleft Sentences ExercisesDo-Cleft & Event Cleft Exercises

Do-Cleft & Event Cleft Exercises

C1 Level

English Exercises is teacher-led by Charlie. About the teacher · How content is made

My first check would be: are you focusing on a person's action or describing an event? For an action, use a do-cleft: What + subject + do/does/did + is/was + verb. Match do, does or did to the original time.

For a whole situation, use What happened was... or What’s happening is.... After is or was, this exercise accepts both the bare infinitive and to + infinitive in do-clefts. Either is fine here 🙌 In event clefts, that before the following clause is optional.