Spent is the past and past participle of spend. Therefore, spend indicates a present action whereas spent indicates a past action. This can be discerned as the main difference between spend Example:- I've already spent my whole salary.

Is it spent or spend?

1. Spent is the past tense and past participle of spend. Spent substances or containers have been used and cannot be used again. Radioactive waste is simply spent fuel.

Did I just spend or spent?

I didn't spend is correct. You must use the infinitive, not the simple past. (The helper verb do takes the simple past already.) The negation of I spent is built by adding the basic form of the verb (in this case spend) to the negation of did.