A) Last night I ____ (lost) my keys – I had to look for them.
B) I ____ (lost) my keys – can you help me look for them?
C) Last night I ____ (lost) my keys – can you help me look for them?
I know that for question A we use Past Simple because we know the specific time the action was completed.
Answer A: Last night I lost my keys – I had to look for them.
For question B we use Present Perfect Simple because the specific time the action was completed is not mentioned, and the consequence of the action is visible in the present moment.
Answer B: I have lost my keys – can you help me look for them?
So! The question C is a combination of question A and B. What tense should we use?
Best Answer
You should use the simple past tense, lost. You know that the losing of the keys happened last night, and that's all that you need to know. Even if you haven't found them yet, it doesn't matter- you lost them last night.
(Fyi, also, when you ask "what tense should we use", you should not include the "to" - you should use the bare infinitive, without the "to", as I am doing in this sentence.)