lận is sentence ending ‘particle’ that works to clarify/correct something for the listener, and emphasise that new information. It’s quite easy to use – but it is very much spoken language, you won’t see it written much.

Here are some examples:

Nay về quê hả? Không, mai mới về lận.

You going to see your folks/go to your hometown today? Nah, actually I’m going TOMORROW.

“Nghĩ cái này khó quá — nó khó tới vậy lận đó hả?

It’s too hard to figure this out… – it’s (actually) that difficult hey?

Ông người Úc nói với em poke có hai nghĩa lận.

An Australian man told me “poke” actually has two meanings (whereas the speaker originally thought it just had one)

Difference with chứ: It differs from chứ because chứ implies the speaker thought the listener already knew that information.