When and how do I use the present continuous tense?

The present continuous tense is used for describing ongoing events or actions that are happening now. Eg. I am going to the shops, He is playing football.

For starters, the present continuous isn't used as much in French as it is in English. In general in French we just use the regular present tense to describe ongoing actions. Eg. Je mange un sandwich can mean both I eat a sandwich but also I am eating a sandwich.

To make it a bit clearer that the action is ongoing you can . . .

1) Add a time phrase. Eg. en ce moment, je mange un sandwich (at the moment, I am eating a sandwich).

OR

2) Use the structure être en train de + infinitive which literally means to be in the middle of doing something. Eg. il est en train de faire ses devoirs (he is in the middle of doing his homework)

There is a present participle (the 'ing' form of the verb in English) in French but it is only used in special circumstances with the phrase en. It is only used to describe two events that are happening at the same time or to emphasise the way in which something happened (while . . .ing or by . . .ing) Eg. Il a quitté la salle en chantant (he left the room while singing), Elle a brisé la fenêtre en lançant un ballon (she broke the window by throwing a ball).

It is really easy to form, you just have to . . .

1) Take the nous form of the present tense (Eg. faire = faisons)

2) Drop the -ons ending (faisons )

3) Add ant to the stem (faisant)

There are (of course!) some irregulars, the most useful ones to remember are . . .

être = étant           savoir = sachant       avoir = ayant