Сетбол был у друга.

Because у normally requires the genitive case.

So:

• друг = nominative, the basic dictionary form
• у друга = at a friend’s place / with a friend / in a friend’s possession

In this sentence, друга is the genitive singular of друг.

У друга can mean something like:

• at a friend’s place
• with a friend
• in a friend’s possession

Russian often uses у + genitive where English might use several different expressions.

So instead of saying literally in the friend, Russian says у друга.

Because Russian uses different prepositions and cases for different ideas.

• у + genitive = by/at someone’s place, with someone, in someone’s possession
• в + prepositional usually means in/inside
• другу is dative, which does not fit after у

So if you want to say something was at a friend’s place or with a friend, the normal pattern is:

• у друга
• у мамы
• у сестры

Был is the past tense form of быть (to be).

Since the sentence is in the past, Russian uses был:

• Сетбол был у друга. = Setbol was at a friend’s place.

In the present tense, Russian usually omits the verb to be:

• Сетбол у друга. = Setbol is at a friend’s place.

So learners often notice that Russian says the past was, but usually leaves out present is.

Because был agrees with Сетбол, which is treated as masculine singular.

Past-tense forms of быть change for gender and number:

• был = masculine
• была = feminine
• было = neuter
• были = plural

So if the subject is masculine singular, you use был.

Yes. Сетбол is the grammatical subject, and был agrees with it.

Structure:

• Сетбол = subject
• был = past-tense verb
• у друга = location/possession phrase

So grammatically the sentence is built like:

[Subject] + [was] + [at a friend’s place / with a friend]

By itself, друга just means of a friend / at a friend’s place.

It does not automatically mean my friend.

Russian often leaves this vague unless context makes it clear. So depending on context, it could be:

• at a friend’s place
• at his friend’s place
• at her friend’s place
• at my friend’s place

If Russian wants to be specific, it can say:

• у моего друга = at my friend’s place
• у его друга = at his friend’s place
• у её друга = at her friend’s place

Because Russian has no articles.

English distinguishes:

• a friend
• the friend

Russian usually does not. It simply says:

• друг
• друга
• у друга

Whether it means a or the depends on context.

Yes. Russian word order is more flexible than English.

For example:

• Сетбол был у друга.
• У друга был сетбол.

Both are possible, but the emphasis changes.

• Сетбол был у друга puts the focus more on Setbol
• У друга был сетбол puts the focus more on the friend / the location / possession

Russian often changes word order for emphasis, topic, or what is already known in the conversation.

Yes. That is very common.

Russian uses у + genitive both for:

• someone physically being at a person’s place
• something being with someone / in someone’s possession

So depending on context, Сетбол был у друга could mean either:

• it was at the friend’s home/place
• the friend had it

Russian often leaves that distinction to context.

Usually:

• Сетбол у друга.

Russian normally omits есть in simple present-tense statements like this.

So:

• Сетбол у друга. = Setbol is at a friend’s place / with a friend.
• Сетбол был у друга. = Setbol was at a friend’s place / with a friend.

That contrast is very useful to notice.