Learn Farsi: من می‌توانستم خوشبخت زندگی کنم.

In Farsi, we use the past continuous tense to express an unreal or hypothetical situation in the present, similar to how English uses 'could' or 'would'. Even though it uses a past tense structure, it translates to the hypothetical 'I could' rather than the factual past 'I was able to'.

Why is the second verb زندگی کنم in the present subjunctive, even though the first verb is in the past continuous?

The modal verb توانستن (to be able to) always requires the following verb to be in the present subjunctive, no matter what tense the modal verb itself is in. Because the main verb is می‌توانستم, the action verb takes the subjunctive form زندگی کنم.

The lesson is about using کاش (I wish) for present unreality. Why isn't it used in this sentence?

While کاش directly states a wish, Farsi uses the exact same grammatical structure (the past continuous tense) to express other unreal or imaginary situations, like 'I could live happily'. You can think of it as a hypothetical statement where a condition like 'If things were different...' is implied.

Is خوشبخت an adjective or an adverb here?

It is functioning as an adverb describing how the person would live (happily). In Farsi, adjectives like خوشبخت (happy or fortunate) are very often used exactly as they are to act as adverbs without needing any special endings.