Although there are some Tagalog adjectives that begin with “ka”, it is more common for adjectives to start with “ma”. The adjective forms of the words you mentioned above are “maganda” (beautiful) and “masaya” (happy), not “kaganda” and “kasaya”. The use of “ka” there stand for “as” and the root words, “ganda” and “saya”, serve as the adjectives, i.e., “kaganda” = as beautiful, “kasaya” = as happy. That’s the reason why such words are usually preceded by the demonstrative pronouns, “ganito” (this), “ganyan” (that), or “ganoon” (that yonder) because these pronouns fill in for the “as this” or “as that” part of the phrase – as beautiful as this/that; as happy as this/that. Malamang = most likely hindi siya = he is not/not him ganun na/ganung/ganoon na/ganoong = (as) that, which is kagandang/kaganda na lalaki = as handsome (beautiful man) Malamang hindi siya ganung kagandang lalaki. = Most likely he is not as handsome as that. – The meaning of “that” (the definition of handsomeness) will depend on whatever the speaker is alluding to, e.g., maybe he is being compared to Brad Pitt. However, if the speaker was referring to handsomeness in its general sense, then the sentence would mean simply, “Most likely he is not that handsome”.

You attach the prefix "ka" if you are trying to express the extent of the adjective you are using without exactly measuring the extent. It's is often associated when something is so or not so <adjective>. For example, "Hindi siya ganoon kataas." Meaning, s/he's not that tall. Meanwhile, the prefix "ma" is used in regular adjectives (ex. malamig or cold, maganda or pretty).

It depends on what do you want to say. Using "ka" instead of "ma" changes the meaning of the sentence. "Malamang hindi siya ganun kagandang lalaki" - Probaby he's not that handsome. (Meaning at some point he is handsome). Using this sentence, at some point you are being polite. But when you say "Malamang hindi siya magandang lalaki" - Probaby he's not handsome. In short you can say "Malamang panget siya" - He's ugly. And this sounds rude. Kasaya is wonderful or extent of being happy Masaya is happy "Gaano kasaya maging masaya?" How wonderful is it to be happy?

In that particular case, ka is different from ma. Ka there means, kasing. It is a shortened form of kasing. It means, not asgood as, or as handsome as... The sentence means, He is not as handsome as you would have expected, or as you would expect. kagandang lalaki is also a shortened form of kasing gandang lalaki. Kasya might be ashortened form of kasing masaya or kasing-saya.

Thanks John ^_^