We also usually use "the" with grammatical terms such as the definite article, the past simple tense and so on. We imagine that there is only one of these things.
• Put these sentences into the present tense.
• You need to use the definite article if the listener knows which one you are talking about.

However, why do these sentecnes use "a"?
- If the noun is unique, use "a definite article".
- Verbs followeb by "an infinitive".
- A past tense.
- You have to use "a semicolon".