You are driving on a two-way road and in front of you drives a vehicle that signals with its left indicator:

You shouldn’t overtake it on a narrow roadway, but you may overtake it from its left side on a wide road.

You may immediately overtake it from its right, by pulling into the “hard shoulder”.

You should take into consideration that the vehicle before you might deviate to the left and you should allow it to do so.

You may overtake after using the horn to warn him. The signal he gives doesn’t oblige you.