[jimerax]

Originally Posted by Silvuh

"Ga" is used to introduce a noun. "Wa" is used when the noun has already been brought up. If you were introducing your umbrella to a conversation, you might say, like,　私のかさがきのうてきだ。 My umbrella is functional. But if, like, someone asked you if your umbrella worked, you'd use "wa" because your umbrella has already been introduced as a noun.　私のかさはきのうてきだ。 sounds more like "As for my umbrella, it's functional."

Actually the sentense "私の傘が機能的だ" contains the meaning of "some other things (like other umbrellas or other possessions of mine) aren't functional". If you just want to say "My umbrella is functional", using "は" is a lot more natural in almost all cases.

In another case, if you want to say "My right leg hurts.", you should use "が" like "(私の)右足が痛い". "は" is awkward in this case.