Learn Polish: Nie lecimy do Londynu, bo są chmury. - We are not flying to London because there are clouds.

Lecimy comes from lecieć, which is the unidirectional verb for flying. We use it here because we are talking about a specific, one-time, one-way trip to a destination (London). If we were saying 'We don't fly to London often', we would use the multidirectional verb: Nie latamy do Londynu.

Why does London change to Londynu?

The preposition do (to) always forces the noun that follows it into the Genitive case. For Londyn (a masculine inanimate noun), the Genitive ending is -u, giving us do Londynu.

Could I use ponieważ instead of bo here?

Yes, you can! Bo and ponieważ both mean 'because'. However, bo is much shorter and by far the most common choice in everyday, casual conversation, while ponieważ sounds a bit more formal.

Why is it just są chmury without a word for 'there'?

In English, we say 'there are clouds' using a filler word 'there'. Polish doesn't use 'there' this way. To say 'there are [plural noun]', you just use the verb są (they are) directly followed by the subject, which in this case is chmury (clouds).