Learn Portuguese (Portugal): A mala está bastante cheia e o casaco não cabe. - The suitcase is quite full and the coat does not fit.

In this sentence, bastante is acting as an adverb modifying the adjective cheia (full), meaning 'quite' or 'rather'. Adverbs do not change their endings to match gender or number. If it were acting as a quantifier modifying a noun, it would change for number (like bastantes malas for 'enough suitcases'), but it never changes for gender.

If the coat doesn't fit, could I say the suitcase is demasiado cheia instead of bastante cheia?

Yes! If you want to say the suitcase is 'too full' rather than just 'quite full', you would use demasiado. Just like bastante in this sentence, demasiado is acting as an adverb here, so it stays masculine and singular even though mala is feminine.

What verb does cabe come from?

It comes from caber (to fit). While cabe (it fits) looks regular for the third-person singular, this verb is highly irregular in the first-person singular, which is caibo (I fit). It is a very useful verb to remember for packing and parking!

Why do we use está instead of é with cheia here?