i guess so but what if i wasn't speaking of a specific grandmother? not this particular grandmother is kind, "a kind grandmother", but "a grandmother is kind" more like grandmothers are or tend to be kind. i guess maybe it's easier to see the difference with an adjective that is always true ... like say you're teaching shapes and you say to a kid "a ball is round" vs. you're on the playground and you say "i need a round snowball" (as opposed to the weirdly shaped one).
I don't understand surly what did you explain.. It is easier than it seems for you.. It's just "a kind grandmother" just like when you use it in English.. See:)
-Do you know this woman.
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she is my grandmother.
-She gave me a gift, she is (a kind grandmother).
أعطتني هدية، إنها (جدة لطيفة)