Discipulus magistro veritatem dicit, quia mendacio non credit.

not

Breakdown of Discipulus magistro veritatem dicit, quia mendacio non credit.

• dicere: to tell
• quia: because
• credere: to believe

Questions & Answers about Discipulus magistro veritatem dicit, quia mendacio non credit.

Why is discipulus in that form, and how do I know it’s the subject?

Discipulus is nominative singular (2nd declension masculine), which is the normal case for the subject of the sentence. Even though Latin word order is flexible, the nominative case is your main clue for “who is doing the action”: the student is the one who dicit and non credit.

Why is magistro in the dative, not the accusative?

With dīcere (to say/tell), Latin commonly uses:

• accusative for the thing said
• dative for the person told

So magistro is dative singular (“to the teacher”), while veritatem is the thing being told.

Why is veritatem accusative?

Veritatem is accusative singular because it’s the direct object of dicit: it’s what the student says/tells.
It comes from vēritās, vēritātis (3rd declension feminine).

Does discipulus magistro veritatem dicit mean “The student says the truth to the teacher” or “The student tells the teacher the truth”? Which is better?

Grammatically it can be taken either way, but Latin structure here strongly matches English :