Learn Farsi: گیاهان شاداب نمی‌مانند، مگر اینکه رطوبت را چک کنید. - The plants will not stay fresh unless you check the humidity.

Yes! The word inke (literally 'this that') is used as a bridge to attach a full clause to a preposition or conjunction. Just as az inke connects 'from' to a clause (from the fact that...), magar inke connects magar (except) to a clause, creating the meaning 'unless' or 'except if'.

Why is the verb چک کنید (chek konid) in the subjunctive mood here?

The phrase magar inke (unless) introduces a condition that hasn't happened yet and might not happen. In Farsi, conditional and uncertain clauses like this require the subjunctive mood.

The English translation says 'will not stay,' but نمی‌مانند (nemimānand) is in the present tense. Why isn't the future tense used?

In Farsi, the present indicative tense is very commonly used to express future actions, especially in 'if/unless' scenarios or when stating general facts and inevitable outcomes. Using the strict future tense would sound unnecessarily formal here.

For the word 'plants,' why is گیاهان (giyāhān) used instead of گیاه‌ها (giyāhhā)? Aren't inanimate objects supposed to use ها?

While ها (-hā) is the standard plural ending for inanimate objects, the ending ان (-ān) is sometimes used for plants, trees, and body parts in formal or written Farsi. گیاه‌ها is also perfectly correct and is what you would hear more often in casual speech.