Learn French: Au fond, il a raison. - Deep down, he is right.

While au fond literally means "at the bottom" (like the bottom of a swimming pool), it is very commonly used figuratively to mean "deep down", "at heart", or "basically". It maps perfectly to how we use "deep down" in English.

Why do we use a (has) instead of est (is) for "is right"?

In French, you "have rightness" rather than "are right". The fixed expression is avoir raison. This is a common pattern where English uses "to be" but French uses avoir, similar to avoir faim (to be hungry) or avoir tort (to be wrong).

Why is there no article before raison? Shouldn't it be il a une raison?

Avoir raison is a fixed idiom, so it drops the article. If you added one and said il a une raison, the meaning would change entirely to "he has a reason" (meaning a motive or an excuse), rather than "he is right".

If I add an opinion verb to this sentence, does it trigger the subjunctive?

It depends on whether you are affirming a belief or expressing doubt. If you say Je pense qu'au fond, il a raison (I think that deep down, he is right), the verb stays in the indicative. However, if you make it negative, you trigger the subjunctive: Je ne pense pas qu'au fond, il ait raison.