I know it sounds a little bit stupid but how about "I think that's afternoon"?
There is a distinction between "tarde" the adverb, which means late, and "tarde" the noun, which means afternoon. To translate "I think it's afternoon" we would say "Eu acho que é de tarde" or "Eu acho que é à tarde"
It would, but "I think that's afternoon" is not so common either. Maybe it could work as an answer to "What comes after 12:00, morning or afternoon?". But even then the Portuguese translation would need an article, "Eu acho que é a tarde". Don't you agree?
Anyway.... the best is sticking to what Duo has proposed =). We gonna go through some crazy thoughts of unresolved situations :S
That would correspond to the Spanish "es de tarde" or "es la tarde". To say "in the afternoon" ins Spanish, we would say "por la tarde".
O audio lento "Eu acho que é tarde" o "é" está com o som sem acento agudo.
I thought penso was 'to think'.....how do you know when to use if either one...or does that not matter?
You can say "Eu penso que é tarde", it has the same meaning. In this context, "eu acho que"/"eu penso que" is a way to express your opinion about something. But the verb "pensar" has a broader meaning, you can use it to say what you're actually (literally) thinking. "Pensar" is the direct translation of "think", while "achar" only means "think" when you're giving your opinion about something.
Why does "que" not mean "that" in this sentence: i.e. I think that is late? Since when does "que" mean "it" and why?
"Que" does not mean "it". That's because "that" was omitted in English here, but it can't be omitted in Portuguese.