Learn Hardcore Japanese: eiga wo mita ato, tomodati to kansou wo hanasimasita. - After watching the movie, I talked about our impressions with a friend.

So 映画を together means the movie (as the thing that was watched).
In English we show this with word order; in Japanese, the particle を shows it.

Why is 見た plain past, but 話しました is polite past? Is that okay?

Yes, this is natural and very common.

In subordinate clauses (like 映画を見たあと = after I watched the movie), the verb is usually in the plain form (dictionary or た-form).
In the main clause (here: 友達と感想を話しました), you choose the politeness level depending on who you are talking to. Here it’s polite past (話しました).

So the pattern is:

映画を見たあと、 → subordinate clause, plain past
友達と感想を話しました。 → main clause, polite past

Mixing plain for the subordinate clause and polite for the main clause is standard Japanese.

What exactly does あと do in 見たあと? Is it a particle? A noun?

あと is originally a noun meaning after / later.

In 映画を見たあと, you have:

見た (past form of 見る, to watch)
- あと (after)

So it literally means the after of having watched the movie → after watching the movie.

A few points:

The verb before あと must be in past (た) form:
- 映画を見るあと ✕
- 映画を見たあと ✓
You can optionally add に:
- 映画を見たあとに、友達と感想を話しました。
  This is also correct, and often used in more formal or written Japanese.
You can also say 映画を見た後で (with 後で) with a very similar meaning: after watching the movie.

What is the role of the comma after あと?

The comma (、) just separates the time/condition clause from the main clause, similar to a comma in English:

映画を見たあと、 → After watching the movie,
友達と感想を話しました。 → I talked about my impressions with my friend.

It’s mostly about readability and rhythm.
You could write it without a comma (映画を見たあと友達と感想を話しました。) and it would still be grammatically correct, just a bit harder to read.

What does と mean after 友達? Is it “with” or “and”?

Here, 友達と means with my friend.

The particle と has several uses:

XとY = X and Y
PersonとV = to do something with someone
- 友達と話す = to talk with a friend
- 家族と住む = to live with family

In this sentence:

友達と → with (my) friend
感想を話しました → talked about impressions

So 友達と感想を話しました = I talked about my impressions with my friend.

Why is 感想 marked with を? Why not 感想について話しました?

Both are possible, but the nuance is slightly different.

感想を話しました。
- Treats 感想 as the direct object: you talked your impressions (i.e., expressed them).
- Natural, a bit more direct and common in everyday speech.
感想について話しました。
- 〜について = about ~
- Literally talked about impressions.
- Emphasizes the topic more; can feel a bit more formal or explanatory.

In many everyday contexts, 感想を話しました is perfectly natural and maybe more common.
If you wanted to stress the topic (for example, in a presentation context), 感想について話しました could be better.

Where is the subject in this sentence? How do we know who did the actions?

The subject (like I, we, he etc.) is simply omitted, which is normal in Japanese.

Grammatically, something like this is understood:

（私は）映画を見たあと、友達と感想を話しました。
→ (I) watched the movie, and after that, I talked about my impressions with my friend.

Japanese often drops pronouns when they are clear from context. From an English point of view, you must mentally supply the subject; here it’s usually I.

Could I say 映画を見てから instead of 映画を見たあと? Is there any difference?

Yes, you can say 映画を見てから、友達と感想を話しました。
The basic meaning is the same: After watching the movie, I talked with my friend about my impressions.

Nuance:

Vたあと
- A bit more neutral, focuses on time after completion.
- Works both in written and spoken Japanese.
Vてから
- Slightly stronger feeling of first do A, then do B (more like a sequence).
- Very common in spoken Japanese.

In many everyday contexts they are interchangeable; learners can treat them as “almost the same” at this level.

Could I say 映画のあと instead of 映画を見たあと?

You can say 映画のあと, but the nuance is slightly different.

映画のあと、友達と感想を話しました。
→ After the movie, I talked about my impressions with my friend.
This focuses on the time after the movie event.
映画を見たあと、友達と感想を話しました。
→ After watching the movie, I talked about my impressions with my friend.
This makes it very clear that you watched it, then did the next action.

Often, context makes both fine, but 映画を見たあと is more explicit about your act of watching.

Why is it 友達と, not 友達に before 話しました?

Both 〜と話す and 〜に話す exist, but they feel slightly different.

友達と話す
- Literally talk with my friend
- Emphasizes conversation / mutual exchange.
友達に話す
- Literally talk to my friend / tell my friend (something)
- Emphasizes one-way telling.

Here, 友達と感想を話しました suggests a two-way conversation about impressions, so と is the natural choice.

Does 友達 mean one friend or several? How do I know?

友達 by itself does not show number. It can mean:

a friend
my friend
friends
my friends

Context decides.
To make it explicit:

一人の友達と = with one friend
友達二人と = with two friends
何人かの友達と = with some friends

In your sentence, without extra words, it could be either a friend or friend(s) depending on the situation.

What is the nuance of 感想 compared with 意見?

Both relate to what you think/feel, but they are used differently:

感想
- Impressions, what you felt / experienced, often after seeing/reading/doing something.
- Typical with movies, books, trips, events.
- Example: 映画の感想 = impressions of the movie.
意見
- Opinion, often more logical / judgmental / argumentative.
- Used in discussions, debates, decisions.
- Example: その問題についての意見 = opinion about that issue.

In this sentence, after watching a movie, sharing your 感想 (impressions) is more natural than 意見, though both can be used in some contexts.

Could I say 感想を言いました instead of 感想を話しました?

Yes, 感想を言いました is also correct, but the nuance changes slightly.

感想を話しました
- 話す = to talk, to speak (often suggests some length / exchange).
- Sounds like you talked about your impressions, maybe as part of a conversation.
感想を言いました
- 言う = to say / to state.
- Focuses more on the act of stating your impressions; could be shorter, more one-directional.

In a casual conversation with a friend after a movie, 感想を話しました feels very natural because it implies a chat about the movie.