There are quite a few ways to write both porque and pode and I’ll go over them with you.
Porque vs Porquê*
When you see porque as one word, it means “because” (which often has the same meaning as ‘pois‘). When the word has a circunflexo on the final letter (ie, porquê), it gives cause, reason or motive to what is being said.
Ex. Ele faltou à prova porque não sabia a data de sua realização (He missed the test because he didn’t know the day it was to be administered).
Ex. Ele não sabia o porquê de suas lágrimas (He didn’t know the reason for his tears).
* – Another way to express cause, reason or motive is to use the word motivo. It is natural to think that using the Portuguese word for reason (razão) would suit your needs but that is incorrect. You will learn how to use razão in good time and with practice.
Por que vs Por quê
Both mean “why” although they can be placed with or without a question mark on the end, just as in English. The rule is that when the word comes at the end of a sentence, it gains the circunflexo on the final letter.
Ex. Por que não compraste aquele livro? (Why didn’t you buy that book?)
Ex. Não compraste aquele livro, por quê? (You didn’t buy that book, why?)
Pode vs Pôde
Coming from the verb poder (to be able to), pode means it/he/she can or is able to. It is the 3rd person present indicative. Pôde, on the other hand, carries the same placement although it is used to speak of the past.
Ex. Ontem, ele não pôde sair mais cedo, mas hoje ele pode (Yesterday, he couldn’t leave earlier, but today he can).