Learn Portuguese (Portugal): Comi metade do bolo.

In Portuguese, the verb ending usually tells you exactly who is doing the action. The ending -i in comi is specifically for the first person singular (I) in the simple past tense. Because of this, you can usually leave out eu, unless you want to emphasize it.

I thought the word for "half" was "meio". Why use "metade" here?

Metade is a noun meaning "a half" or "one half", which you use when talking about dividing a physical object like a cake. You use meio when "half" acts as an adjective describing a noun (like meio quilo for half a kilo) or as an adverb meaning "kind of" (like meio cansado for kind of tired).

Why is it "do bolo" instead of just "de bolo"?

Do is a mandatory contraction of the preposition de (of) and the masculine definite article o (the). If you just said de bolo, it would mean "half of cake" in a general, abstract sense. Because you are talking about a specific cake, you need the "the", making it de + o = do.

How do I know if "comi" is past or present tense?