Learn Hardcore Russian: Вчера мы посмотрели первый эпизод нового сериала. - Yesterday we watched the first episode of a new series.

посмотрели is perfective (from посмотреть). It focuses on the completed result: we watched it (and finished it).
смотрели is imperfective (from смотреть). It focuses on the process or duration: we were watching / we used to watch / we watched (but don’t emphasize finishing).

In this sentence, the idea is that the first episode was watched completely, so perfective посмотрели is natural:
Вчера мы посмотрели первый эпизод… = Yesterday we watched (and finished watching) the first episode…

How is the past tense form посмотрели built from the verb посмотреть?

Infinitive: посмотреть (perfective).
Remove the infinitive ending -ть → посмотре-.
For past tense, you add endings based on gender/number:
- masculine singular: посмотрел
- feminine singular: посмотрела
- neuter singular: посмотрело
- plural (any gender): посмотрели

Since мы (we) is plural, you use посмотрели.

What case is первый эпизод, and why does it look like the dictionary form?

первый эпизод is in the accusative singular, because it’s the direct object of the verb посмотрели (we watched what? → the first episode).

эпизод is a masculine, inanimate noun.
For masculine, inanimate nouns in singular, the accusative form = nominative form.
первый is a masculine adjective in the nominative/accusative singular, agreeing with эпизод.

So:

Nominative: первый эпизод
Accusative (inanimate): первый эпизод (same form)

Why is it нового сериала and not новый сериал?

Because нового сериала depends on эпизод and answers “episode of what?”:

the first episode of a new series → первый эпизод нового сериала

This is a typical genitive relationship: X of Y → родительный падеж (Genitive).

Noun: сериал → genitive singular: сериала
Adjective: новый (new) → genitive masculine singular: нового

So both words are in genitive singular: нового сериала = of a new series.

How can I tell that нового сериала is genitive case?

By the endings:

Masculine noun сериал:
- Nominative: сериал
- Genitive: сериала (ends in -а)
Adjective новый:
- Nominative masculine: новый
- Genitive masculine: нового (ends in -ого)

The pattern -ого (adjective) + -а (noun) is very common for genitive masculine singular.

Could the sentence mean “We were watching the first episode” (a process), not that we finished it?

Not with посмотрели. Perfective посмотрели strongly suggests you completed watching it.

To focus on the process (like English “were watching”), you would use the imperfective:

Вчера мы смотрели первый эпизод нового сериала.
= Yesterday we were watching / spent time watching the first episode of a new series.
(It doesn’t state clearly whether you finished it.)

To emphasize finishing partway through or up to some point, Russians often use:

досмотрели (finished watching),
посмотрели до конца (watched to the end).

Does посмотрели translate more like “watched” or “have watched”?

It can correspond to both English forms, depending on context. Russian past tense doesn’t distinguish simple past vs. present perfect.

Вчера мы посмотрели первый эпизод…
→ Yesterday we watched the first episode… (usual translation)
Мы уже посмотрели первый эпизод.
→ We have already watched the first episode.

Russian relies on context and adverbs like вчера, уже, только что to express nuances that English expresses with watched vs have watched.

Can Russian leave out мы here, like in Spanish or Italian?

Yes, you can often omit the subject pronoun in Russian when it is clear from the verb ending:

Вчера посмотрели первый эпизод нового сериала.

The verb посмотрели is past plural, so it usually implies мы (we) or sometimes они (they), depending on context. In everyday speech, people commonly drop мы if it’s obvious who is meant.

However, including мы is perfectly natural and neutral; it can make the subject clearer or slightly emphasized: мы, not someone else.

Is the word order fixed? Could I say Мы вчера посмотрели… or Мы посмотрели вчера…?

Russian word order is fairly flexible. Common variants include:

Вчера мы посмотрели первый эпизод нового сериала.
Slight emphasis on вчера (yesterday is the setting).
Мы вчера посмотрели первый эпизод нового сериала.
Very neutral; many learners and natives would say this.
Мы посмотрели вчера первый эпизод нового сериала.
Also possible; the focus shifts a bit toward посмотрели (the action).

All three are grammatically correct. The default “neutral” version many people might use is:

Мы вчера посмотрели первый эпизод нового сериала.

Why is there no word for “the” in “the first episode”? How do I know it’s not “a first episode”?

Russian has no articles (a, an, the), so definiteness is inferred from:

Context – you and the listener both know which series & which episode you’re talking about.
Words like “first” – первый almost automatically suggests the in English: there is usually only one “first” episode of a given series.
Position and known information – if you already mentioned the series, первый эпизод нового сериала will naturally be understood as the first episode of a new series.

So:

первый эпизод нового сериала → usually translated as “the first episode of a new series”, not “a first episode”.

What’s the difference between эпизод and серия? Which is more natural here?

Both can mean “episode”, but usage differs:

серия – the most common everyday word for a TV episode:
- первая серия нового сериала = the first episode of a new series.
эпизод – more bookish/technical or used in specific contexts (film, narrative segments, etc.). It’s used, but серия is more typical for regular TV shows.

So many natives would naturally say:

Вчера мы посмотрели первую серию нового сериала.

Does сериал automatically mean a TV series, or do I need to say something like “телесериал”?

Nowadays сериал by default means a TV series / streaming series (or web series). You don’t need to say телесериал unless you specifically want to stress that it is on TV.

So:

новый сериал = a new TV show / new series.
первый эпизод нового сериала = the first episode of a new TV series.

How do I pronounce and where is the stress in the key words of this sentence?

Stresses (marked with ´ over the stressed vowel):

вчера́ – vcherá (stress on -ра)
мы – my (short, unstressed in the sentence rhythm)
посмотре́ли – posmotrélʹi (stress on -тре-: пос-мо-тре-ли)
пе́рвый – pér-vyj (stress on пе)
эпизо́д – e-pi-zót
но́вого – nó-va-va (NO-va-va; stress on the first о)
сериа́ла – se-ri-á-la

Putting it together (stressed syllables in caps for clarity):

vche-RA, my, posmo-TRÉ-li, PÉR-vyj e-pi-ZÓD NO-va-va se-ri-Á-la.