How do French people convert a declarative sentence into an interrogative one?

 1. Using Est-ce-que...

Est-ce que... is very commonly used.

• Tu participes à toutes nos réunions.
  You take part in all our meetings.
• Est-ce que tu participes à toutes nos réunions ?
  Do you take part in all our meetings?

• Il joue au football.
  He plays soccer.
• Est-ce qu'il joue au football ?
  Does he play soccer?

• Marie aime la peinture.
  Marie is fond of painting.
• Est-ce que Marie aime la peinture ?
  Is Marie fond of painting?

• Nos amis préparent le dîner.
  Our friends are preparing the dinner.
• Est-ce que nos amis préparent le dîner ?
  Do our friends prepare the dinner?

 2. Reverting the subject and the verb

• Tu participes à toutes nos réunions.
  You take part in all our meetings.

becomes:

• Participes-tu à toutes nos réunions ?
  Do you take part in all our meetings?

Note: a dash must be included between the verb and the subject.

• Il joue au football.
  He plays soccer.
• Joue-t-il au football ?
  Does he play soccer?

Note: add -t- between the verb and the subject when the verb ends with a vowel and the subject begins with a vowel.

• Marie aime la peinture.
  Marie is fond of painting.
• Marie aime-t-elle la peinture ?
  Is Marie found of painting?

Note: You can't say : "Aime Marie la peinture ?" Instead, replace the firstname by the corresponding pronoun and keep the firstname at the beginning of the phrase. Here is a last example with the same rule:

• Nos amis préparent le dîner.
  Our friends are preparing the dinner.
• Nos amis préparent-ils le dîner ?
  Do our friends prepare the dinner?

 3. Changing your voice tone

Even easier! Just add a question mark and change your voice tone.

Let's get the examples we used in the previous pages:

• Tu participes à toutes nos réunions.
  You take part in all our meetings.
• Tu participes à toutes nos réunions ?
  Do you take part in all our meetings?

• Il joue au football.
  He plays soccer.
• Il joue au football ?
  Does he play soccer?

• Marie aime la peinture.
  Marie is fond of painting.
• Marie aime la peinture ?
  Is Marie found of painting?

• Nos amis préparent le dîner.
  Our friends are preparing the dinner.
• Nos amis préparent le dîner ?
  Do our friends prepare the dinner?