It can be used in both the present and past tense, as well as in various grammatical forms (e.g., "improvising," "improvised," "improvises," "improvise").
For example:
* Present Tense: "The musician improvises a solo."
* Past Tense: "The chef improvised a new dish using what was in the fridge."
* Present Participle: "The actors are improvising their lines."
* Past Participle: "She improvised a solution to the problem."