Learn Turkish: Birkaç marka denedim.

Birkaç is a determiner meaning "a few" and is placed directly before a noun, like birkaç marka (a few brands). Birkaçı is a pronoun meaning "a few of them". If you wanted to say "I tried a few of the brands", you would use the pronoun form and say Markaların birkaçını denedim. Because this sentence just says "a few brands", we use the simpler word birkaç.

Since birkaç means "a few", why isn't the word for brand plural (markalar)?

In Turkish, whenever you use a quantity word that implies more than one (like birkaç for "a few", çok for "a lot", or numbers), the noun that follows it must stay singular. Therefore, it is always birkaç marka, and never birkaç markalar.

Why doesn't marka have an accusative ending here?

You only use the accusative case (endings like -yı or -i) for specific, definite direct objects (like "the brand"). Because you tried "a few brands" in general, rather than specific brands you were already talking about, it remains indefinite and stays in its plain dictionary form.