Learn Hardcore Russian: Вчера мы смотрели замечательный фильм о путешествии по морю. - Yesterday we watched a wonderful movie about a journey by sea.

смотрели is the past tense of imperfective смотреть.
It presents the action as a process: we were watching / we spent time watching / we watched (neutral statement of what we did).
посмотрели is the past tense of perfective посмотреть.
It focuses on the completion or result: we watched it (and finished it / got it over with / ended up having watched it).

In a neutral narrative about what you did yesterday, смотрели is very natural.
If you want to emphasize that you finished watching the film, or that this completed action is somehow important for what follows, you might use посмотрели:

Вчера мы посмотрели фильм. – We watched the film (from start to finish / and now that’s done).

How is the past tense form смотрели formed from смотреть?

For most Russian verbs, the past tense is formed like this:

Take the infinitive: смотреть.
Remove -ть → смотре-.
Add л → смотрел (basic past tense stem).
Add gender/number endings:

смотрел – he watched (masculine singular)
смотрела – she watched (feminine singular)
смотрело – it watched (neuter singular)
смотрели – they / we / you (plural) watched

Because the subject is мы (we), you need the plural form смотрели.

Could the word order be Мы вчера смотрели замечательный фильм… instead of Вчера мы смотрели…? Does the word order change the meaning?

Yes, you can say both:

Вчера мы смотрели замечательный фильм…
Мы вчера смотрели замечательный фильм…

Both are grammatically correct and mean essentially the same thing.

Subtle nuance:

Вчера мы… – starts with вчера, slightly emphasizing when it happened (yesterday is the frame).
Мы вчера… – starts with мы, slightly emphasizing who did it (we are the topic).

Russian word order is more flexible than English. The basic information (we, yesterday, watched, a wonderful film) stays the same; the order mostly changes what is highlighted. In everyday speech, both versions are very common.

Why is замечательный before фильм? Could you put it after the noun?

In Russian, adjectives normally come before the nouns they modify:

замечательный фильм – a wonderful film
интересная книга – an interesting book

Putting the adjective after the noun – фильм замечательный – is possible but less neutral. It usually:

sounds more evaluative, like a comment:
Фильм замечательный. – The film is wonderful. (a statement/judgment)
is not used inside a noun phrase in the same way as in English (a film wonderful is wrong in both languages as a pure noun phrase).

So in this sentence, замечательный фильм is the normal, neutral order for a wonderful film.

Why is it фильм, not some other case form? Is фильм in the nominative or accusative here?

Grammatically, фильм is in the accusative singular, because it is the direct object of смотрели:

Who watched? – мы (nominative, subject)
Watched what? – фильм (accusative, direct object)

For inanimate masculine nouns, the nominative and accusative singular forms are identical:

Nominative: фильм
Accusative: фильм

So even though it looks like nominative, its function in the sentence is accusative: it receives the action of watching.

Why is it о путешествии, not о путешествие? What case is used after о?

The preposition о (about) usually takes the prepositional case.

The noun путешествие (journey, trip) is neuter, ending in -ие. Its singular forms:

Nominative: путешествие (a journey)
Prepositional: путешествии (about a journey, in a journey, etc.)

So о путешествии literally means about (the) journey / about a journey.

Rule: after о (in the meaning about), use prepositional:

о книге – about the book
о маме – about mom
о путешествии – about the journey

Why does путешествии end with -ии and not just -и?

For many neuter nouns ending in -ие, the prepositional singular ends in -ии:

здание → в здании – in the building
море → о море (here it keeps е, different pattern)
путешествие → о путешествии – about the journey

For путешествие, the pattern is:

путешествие (nom.)
путешествии (prep.)

So the -и ending plus the extra -и you see is just the regular prepositional ending -ии for this type of noun. It doesn’t change pronunciation very much; the stress stays earlier in the word: путЕшествии.

Why is it по морю and not по море? What case does по use here?

Here по means over / across / by (sea) and it takes the dative case.

The noun море (sea) is neuter. Its dative singular is морю:

Nominative: море
Dative: морю

With the meaning of movement/on a surface, по + dative is standard:

по дороге – along the road
по реке – along the river / on the river
по морю – across/by sea

So по морю literally means over/across/on the sea, which is often translated as by sea or on the sea.

What is the difference between о путешествии по морю and о путешествии на море?

They suggest different images:

о путешествии по морю
– about a journey over/across the sea, usually by ship or boat, moving on the water.
о путешествии на море
– about a trip to the sea / to the seaside (e.g., going on holiday to the seaside resort), not necessarily traveling on the water.

So по морю focuses on traveling on the sea itself; на море focuses on going to the sea (as a place).

Can you replace о with про in фильм о путешествии по морю? What would change?

Yes, you can say:

фильм о путешествии по морю
фильм про путешествие по морю

They both mean a film about a journey by sea, but there is a stylistic difference:

о is more neutral / standard / literary.
про is more colloquial / informal, very common in everyday speech.

So in casual conversation, фильм про путешествие по морю sounds very natural.
In more formal writing or speech, фильм о путешествии по морю is preferred.

Could мы be omitted, like Вчера смотрели замечательный фильм…? Would that be correct?

Yes, it is grammatically possible to omit мы, because the verb ending -ли already shows that the subject is plural (we/they/you-plural).

However:

Вчера смотрели замечательный фильм… without context is a bit vague: Who watched? we? they? you (plural)?
In conversation, if it is already clear from context that we are being discussed, this can be perfectly natural.

Russian often drops personal pronouns when the subject is obvious, but if you want to be clear and neutral, especially in simple learner sentences, including мы is better: Вчера мы смотрели….

How would the verb change if it were I watched or he/she watched instead of we watched?

Using the same verb смотреть (imperfective), past tense:

я смотрел – I watched (male speaker)
я смотрела – I watched (female speaker)
он смотрел – he watched
она смотрела – she watched
оно смотрело – it watched (for neuter nouns)
мы смотрели – we watched
они смотрели – they watched

So your sentence could be:

Вчера я смотрел замечательный фильм… (said by a man)
Вчера я смотрела замечательный фильм… (said by a woman)
Вчера он смотрел замечательный фильм… (he)
Вчера она смотрела замечательный фильм… (she)

How would you make this sentence negative: we did not watch a wonderful film…?

To make it negative, add не before the verb:

Вчера мы не смотрели замечательный фильм о путешествии по морю.

This means Yesterday we did not watch a wonderful film about a journey by sea (for example, if you are denying or correcting someone).

You can also change intonation or context to emphasize what exactly you are denying (yesterday / we / this specific film, etc.), but grammatically the only required change is adding не before смотрели.

Where is the main stress in вчера, смотрели, and замечательный?

Approximate stress (capitalized syllable is stressed):

вчера – vchera → вчерА
смотрели – smotreli → смотрЕли
замечательный – zamechatelnyj → замечАтельный

So spoken naturally:

Вчера мы смотрЕли замечАтельный фильм о путешествии по мОрю.
(also note: мОрю is stressed on мо-)