Marie touche son nez quand elle réfléchit à une nouvelle phrase.

In French, possessive adjectives (son, sa, ses) agree with the gender and number of the noun owned, not with the owner.

• nez is masculine singular → you must use son.
• So:
  • son nez = her/his nose (because nez is masculine)
  • sa main = her/his hand (because main is feminine)

Context tells you whether son means his or her. Here, we know it’s her nose because the subject is Marie.

Yes, and Marie se touche le nez is actually more idiomatic French.

• Marie touche son nez: grammatically correct, but it could, in theory, mean she touches someone else’s nose (even if context suggests it’s her own).
• Marie se touche le nez: clearly means Marie touches her own nose, thanks to the reflexive pronoun se and the definite article le.

French usually uses:

• se + verb + definite article + body part
  e.g. Elle se lave les mains, Il se brosse les dents.

So the most natural version is: Marie se touche le nez quand elle réfléchit à une nouvelle phrase.

The French présent covers both:

• English simple present: She touches her nose
• English present continuous: She is touching her nose

Context decides which English form you choose.
Here, because it describes a habit, good translations are:

• Marie touches her nose when she is thinking about a new sentence.
• Or: Marie touches her nose when she thinks about a new sentence.

The verb réfléchir is normally followed by à:

• réfléchir à quelque chose = to think carefully about something, to consider something.

You could also say:

• Marie pense à une nouvelle phrase (more general: “thinks of / about a new sentence”).

réfléchir sur exists but is rarer and more formal, often used for more abstract or academic topics (e.g. réfléchir sur un concept philosophique).

So in everyday French, the natural choice here is réfléchir à.

The à is required by the verb réfléchir. It’s not optional:

• réfléchir à quelque chose = to think about something.

So the structure is:

• réfléchir à + noun → réfléchir à une nouvelle phrase

You generally can’t replace this à with de or sur in normal speech when you use réfléchir in this sense.

You use the pronoun y, which replaces à + thing (not a person):

• Marie touche son nez quand elle réfléchit à une nouvelle phrase.
• → Marie touche son nez quand elle y réfléchit.
  (y = à une nouvelle phrase)

Note the pronoun position: in a simple tense, y goes before the conjugated verb:

• elle y réfléchit

Yes, you can say:

• Marie touche son nez lorsqu’elle réfléchit à une nouvelle phrase.

quand and lorsque often mean the same thing (“when”).
Differences:

• lorsque is a bit more formal/literary.
• Before elle, you must write lorsqu’elle (with an apostrophe) for euphony.

In everyday spoken French, quand is more common, but both are correct here.

In French, you must include the subject pronoun with a conjugated verb:

• quand elle réfléchit ✅
• quand réfléchit ❌ (ungrammatical)

French is not a “pro-drop” language like Spanish or Italian; you can’t normally omit subject pronouns.
Only in the imperative (commands) do you drop them, e.g. Réfléchis !

Many common adjectives in French usually come before the noun, including nouveau / nouvelle.

• une nouvelle phrase is the normal order and usually means a new (another/additional) sentence.

You can say une phrase nouvelle, but it changes the nuance:

• une phrase nouvelle suggests the phrase itself is new in nature, original, or innovative in style.

So:

• une nouvelle phrase = another/new sentence in the sequence.
• une phrase nouvelle = a sentence that is new/original in character.

This is a classic false friend.

• French une phrase = a sentence in English (a complete unit with a verb, etc.).
• English a phrase = a group of words, often not a full sentence.

So une nouvelle phrase here means a new sentence, not a new phrase in the English grammatical sense.

Yes, both come from réfléchir (to think, to reflect). It’s a regular -ir verb of the second group. Present tense:

• je réfléchis
• tu réfléchis
• il / elle / on réfléchit
• nous réfléchissons
• vous réfléchissez
• ils / elles réfléchissent

In the sentence, elle réfléchit is the 3rd person singular form.