Learn Farsi: ساقه‌ی این گیاه ماندنی نیست، ولی برگ آن خوردنی است. - The stem of this plant is not worth keeping, but its leaf is edible.

By adding -i (ی) to a full infinitive, you create an adjective that describes worth, capability, or fitness. Here, khordan (to eat) becomes khordani (خوردنی), meaning edible or fit to be eaten. Similarly, māndan (to stay/remain) becomes māndani (ماندنی), meaning capable of lasting or worth keeping.

The verb māndan (ماندن) usually means to stay or remain. Why does māndani nist (ماندنی نیست) translate to "not worth keeping"?

While māndan means "to stay," its adjective form māndani (ماندنی) implies something has the durability to last or stick around. In the context of a plant's stem, saying it is māndani nist means it won't last, which is naturally translated into English as "not worth keeping."

Why does sāqe (ساقه) have the letter ye (ی) attached to it in sāqe-ye (ساقه‌ی)?

This is the Ezafe, the grammatical link used to connect a noun to its possessor or adjective. Because sāqe (stem) ends in a "silent heh" (ه) which makes an "e" sound, we must add a visible ye (ی) to act as a bridge before pronouncing the Ezafe.

The English translation says "its leaf". Why does the sentence use barg-e ān (برگ آن) instead of the possessive suffix barg-ash (برگش)?