Thought (wrongly): tưởng

Tưởng means "thought", but only when the thought turned out wrong: Em tưởng hôm nay là thứ bảy. I thought today was Saturday (and it isn't). For a thought that may well be right, use nghĩ or thấy: Em nghĩ hôm nay là thứ bảy, I think it's Saturday.

The thought follows straight after, sometimes with là: Tôi tưởng là anh về rồi. Cứ tưởng means "had assumed all along": Tôi cứ tưởng chị là người Hà Nội. I'd always assumed you were from Hà Nội. In Saigon it often comes after ủa, the little word of surprise: Ủa, em tưởng anh về rồi. Oh, I thought you'd gone home.

Tưởng gì! on its own means "Oh, is that all?": you thought it was something serious, and it isn't.

Don't confuse it with tưởng tượng, "imagine".