Will I lose money on ETF?

Yes you might. It is not a capital guaranteed product.

As with all equity related products, yes if you are selling below what you bought at.

Which is why it's important to have liquidity. When markets tank and things are in the red in the short term, you do not want to be in a position where you feel like you are forced to sell; you want to be able to hold it for the long term and get a positive return in the long run.