固然【gùrán】

固然 agrees that something is true or good, then the sentence turns with 但是, 可是, 不过 or 却 to the point the speaker cares about more. It is an adverb and sits after the subject, in front of the predicate.

After the subject, not before it

固然 can't open the sentence ahead of the subject the way 虽然 can.

• × 固然他很聪明，但是不努力 → 他固然很聪明，但是不努力

固然 or 虽然

虽然 just sets up a contrast. 固然 adds that the speaker accepts the first point as valid – it is true, it does matter – and is saying it isn't the whole story. That makes 固然 the word for weighing two considerations in an argument. It is neutral to written; in casual speech 虽然 is more usual. The full 固然…但是 frame for longer arguments has its own point.

Both are fine: 固然…也

With 也 in the second half, there's no turn at all. The speaker approves of the first option and says the second is acceptable too.