Learn Hardcore French: Il dit qu’un jour, ils se souviendront de cette période comme d’un moment profond de leur vie. - He says that one day, they will remember this period as a profound moment in their lives.

So ils se souviendront literally is “they will remember themselves,” but in normal English we just say “they will remember.” You always need that little se with souvenir: you cannot say ils souviendront on its own.

How is se souviendront formed? Why is it not souveniront or something simpler?

Souvenir is an irregular verb. Its future stem is souviendr-, not souvenr- or souvenir-. The future tense is:

stem souviendr-
- future endings (-ai, -as, -a, -ons, -ez, -ont)

For ils:

ils se souviendront = se (reflexive pronoun for ils) + souviendr- (future stem) + -ont (future ending for ils).

So:

infinitive: se souvenir
future: ils se souviendront (“they will remember”)

Why is it Il dit qu’un jour, ils se souviendront… with the future tense, not something like a subjunctive or a present?

After dire que, French normally uses the indicative, not the subjunctive, because this verb just reports speech; it does not express doubt, emotion, or necessity.

Here, the action is in the future relative to the moment of speaking:

Il dit (he says now)
qu’un jour, ils se souviendront… (that one day, in the future, they will remember…)

So the natural tense is the future simple: ils se souviendront.

If you used the present (ils se souviennent), it would mean “they remember (now),” which does not match un jour (“one day / someday”).

Why is there qu’un instead of que un?

This is just elision, a normal spelling rule in French.

que
- un → qu’un

When que comes before a word starting with a vowel sound (un, homme, ami, etc.), the final e of que disappears and is replaced with an apostrophe:

qu’un, qu’homme, qu’ami

It doesn’t change the meaning at all; it only makes pronunciation smoother.

Why is it se souviendront de cette période? Why do we need de before cette période?

The verb se souvenir is always followed by de:

se souvenir de quelque chose / de quelqu’un

Examples:

Je me souviens de ce film.
Nous nous souvenons de toi.

So in this sentence:

ils se souviendront de cette période
= “they will remember this period”

You cannot drop de: se souvenir cette période would be incorrect.

Why do we have de twice: de cette période comme d’un moment profond?

The structure here is:

se souvenir de A comme de B

Meaning: “to remember A as B,” “to remember A as being B.”

So:

de cette période = of this period
comme d’un moment profond de leur vie = as a profound moment in their life

The second de appears because we are still governed by se souvenir de; we are basically saying:

“remember this period as (being) one of the moments of their life.”

This pattern se souvenir de X comme de Y is very common in French.

Could we say comme un moment profond instead of comme d’un moment profond? Would that be wrong?

Comme un moment profond is not wrong in itself, but it slightly changes the structure:

se souvenir de cette période comme d’un moment profond
puts more emphasis on “remember this period as (one) of the profound moments of their life.” It keeps the link with de from se souvenir de and feels very idiomatic.
se souvenir de cette période comme un moment profond
can sound closer to “remember this period as a profound moment,” more like a simple predicate without repeating de.

In practice:

de cette période comme d’un moment profond is the more typical and natural pattern with se souvenir de.
comme un moment profond is possible, but the given version sounds more elegant and idiomatic.

Why is it un moment profond and not une moment profonde? How is the adjective profond agreeing here?

Adjectives in French agree in gender and number with the noun they describe.

moment is masculine singular: un moment
Therefore the adjective must also be masculine singular: profond

So:

un moment profond (masculine singular)
If it were feminine: une période profonde, une expérience profonde

You might be tempted by profonde because période is feminine (cette période), but profond is describing moment, not période.

Why is it de leur vie and not de leurs vies, since ils is plural?

In French, when talking about abstract things like life, future, childhood, etc., it is very common to keep them singular, even if the people are plural:

un moment important de leur vie = “an important moment in their life”
à un certain moment de leur carrière = “at a certain point in their career”

Leur before a singular noun means “their (shared or respective) …”:

leur vie = their life / their lives (depending on context)
leurs vies is possible but usually implies strongly distinct, separate lives and is much less common in this type of abstract expression.

Here, de leur vie is the natural, idiomatic choice.

Why is there a comma after un jour? Could we move un jour to another place in the sentence?

Un jour here is an adverbial expression of time (“one day / someday”).

The comma:
- The comma after un jour is optional but very common.
- It separates the time phrase from the rest of the clause for clarity:
  Il dit qu’un jour, ils se souviendront…
Position: You can move un jour just like “one day” in English:
- Il dit qu’un jour, ils se souviendront…
- Il dit qu’ils se souviendront un jour de cette période…
- Un jour, il dit qu’ils se souviendront de cette période… (slightly different emphasis)

All are grammatically possible; the original version highlights un jour more strongly.

Could we use se rappeler instead of se souvenir de here? Would the grammar change?

Yes, you could use se rappeler, but the construction is different:

se souvenir de quelque chose
se rappeler quelque chose (no de with a direct object)

So:

With se souvenir:
Ils se souviendront de cette période.
With se rappeler:
Ils se rappelleront cette période.

In your full sentence:

Il dit qu’un jour, ils se rappelleront cette période comme un moment profond de leur vie.

Notice:

With se rappeler, if you keep comme d’un moment, it sounds a bit heavy; comme un moment profond is more natural in that version.
With se souvenir de, de cette période comme d’un moment profond is the classic pattern.

Both verbs are common, but se souvenir de feels slightly more neutral and is very frequent in written French.

If we changed the first verb to the past, like Il a dit or Il disait, would ils se souviendront stay in the future, or would it become ils se souviendraient?

Both are possible, but the nuance changes:

Il a dit qu’un jour, ils se souviendront de cette période…
- Still uses the future (se souviendront).
- This keeps the future as seen from the speaker’s perspective (now).
- Common in everyday speech.
Il a dit / Il disait qu’un jour, ils se souviendraient de cette période…
- Uses the future-in-the-past (conditional: se souviendraient).
- This is more “textbook” sequence of tenses: their remembering is future relative to that past moment of speaking.
- Often a bit more formal or literary.

So grammatically:

se souviendront and se souviendraient can both be correct; the choice depends on perspective and style.