Learn French: Je vais dire la vérité. - I am going to tell the truth.

Because it follows another conjugated verb. In French, when two verbs are used together to form a single idea, the first verb (vais) is conjugated to match the subject, and the second verb (dire) stays in its infinitive (dictionary) form.

Does dire mean "to say" or "to tell"?

It means both! English has different rules for "say" and "tell", but French uses dire for both. In this sentence, dire la vérité naturally translates to "to tell the truth".

Why does this sentence use vais instead of just a future tense verb?

Using a conjugated form of aller (here, vais) plus an infinitive creates the near future tense, which translates to "going to do something". It is very common in everyday French for actions that are planned or happening soon.

Is there a trick to remembering that vérité is a feminine noun (la)?

Yes! Most French abstract nouns ending in -té (like la beauté, la liberté, and la vérité) are feminine. They very often correspond to English words ending in "-ty".