Learn French: Tu ne dis jamais la vérité.

Dis is the tu (informal you) form of the irregular verb dire (to say or to tell) in the present tense. For the singular forms, the pattern is: je dis, tu dis, il/elle/on dit.

Why does dis translate to 'tell' here instead of 'say'?

In French, the verb dire covers both 'to say' and 'to tell'. While English makes a distinction between the two, French just uses dire for both concepts.

Why isn't there a pas in this sentence?

The structure ne ... jamais means 'never'. It wraps around the conjugated verb just like ne ... pas, but it replaces the pas completely. Saying ne dis pas jamais would be incorrect.

Would a French speaker ever drop the ne when saying this out loud?

Yes! In everyday spoken French, it is very common to drop the ne in negative sentences. You will often hear people simply say Tu dis jamais la vérité.