В прошлом году дедушка видел оленя у реки, а однажды даже медведя.

Because дедушка means grandpa, so it refers to a male person. In Russian, some nouns for male people have endings that look like typical feminine nouns, but they still take masculine agreement.

That is why the verb is видел and not видела.

• дедушка видел = grandpa saw
• бабушка видела = grandma saw

So the ending of the noun does not always determine real gender agreement by itself; meaning matters too.

This is the normal Russian way to say last year in a time expression.

• в = in
• прошлом = last / past
• году = year, in the special prepositional/locative form used here

The noun год is a little irregular. In expressions like в этом году, в прошлом году, в следующем году, Russian uses году, not годе.

So you should learn в прошлом году as a standard time phrase.

Because they are direct objects of видел, and both nouns are masculine animate nouns.

With masculine animate nouns in Russian:

• nominative: олень, медведь
• accusative: оленя, медведя

Russian makes the accusative of masculine animate nouns look like the genitive.

So:

• дедушка видел оленя
• дедушка видел медведя

If the noun were masculine and inanimate, the accusative would usually look like the nominative instead.

Реки is genitive singular of река.

It comes after у, which here means by, near, or at. In Russian, у takes the genitive case.

• река = river
• у реки = by the river / near the river

So the phrase literally works like near the river.

Because Russian often omits repeated words when they are easy to understand from context.

The full version would be:

В прошлом году дедушка видел оленя у реки, а однажды даже видел медведя.

But repeating видел is unnecessary, so Russian naturally leaves it out.

This is a very common kind of ellipsis. English does something similar sometimes:

• He saw a deer by the river, and once even a bear.

Видел presents the fact or experience of seeing, not a single completed event viewed as one whole.

Here the sentence is talking about what grandpa happened to see during last year as a general period:

• he saw a deer by the river
• and once he even saw a bear

That makes видел sound natural.

If you used увидел, it would focus more on one specific completed moment of spotting something. That would change the nuance.

Very roughly:

• видел = saw / had seen / was in the state of having seen
• увидел = caught sight of / saw at a particular moment

Here а is best understood as a linking word with a slight contrast or shift. It is somewhere between and and but, depending on context.

In this sentence, it feels like:

• and, on one occasion, even a bear
• or and once even a bear

The contrast is mild: seeing a deer is one thing, but seeing a bear is more surprising.

That is why а works well here. Russian uses а very often where English might simply use and, or sometimes but.

Because а is a coordinating conjunction joining two clause-like parts, and in Russian it is normally preceded by a comma.

Even though the second part leaves out the verb, it still counts as a separate coordinated part of the sentence:

• ... дедушка видел оленя у реки, а однажды даже медведя.

So the comma is standard.

They add emphasis and surprise.

• однажды = once / one day / on one occasion
• даже = even

So однажды даже медведя means something like:

• and once even a bear
• and on one occasion even a bear

The word даже highlights медведя as unexpected or impressive. It tells the listener that a bear is more surprising than a deer.

Russian word order is more flexible than English word order, but the chosen order affects emphasis.

In this sentence:

• В прошлом году sets the time first
• дедушка gives the subject
• видел оленя у реки gives the main event
• а однажды даже медведя saves the most surprising part for the end

Placing медведя at the end makes it sound especially striking. The sentence builds up to that final word.

So the order is not random; it helps the sentence sound natural and expressive.