Перед сном я положил пакетик чая в кружку.

Literally, Перед сном means before sleep.

In natural English, it often corresponds to:

• before bed
• before going to sleep
• before sleeping

So it is a very normal Russian way to talk about something that happens shortly before bedtime.

Because the preposition перед requires the instrumental case.

So:

• сон = sleep
• перед сном = before sleep

This is a pattern worth remembering:

• перед домом = in front of the house
• перед уроком = before the lesson
• перед сном = before sleep

The same preposition can be spatial (in front of) or temporal (before), but it still takes the instrumental.

Положил is the perfective past form of положить, so it presents the action as completed: the speaker put the teabag into the mug, and that action is finished.

Compare:

• положил = put, placed down, completed the act
• клал = was putting / used to put / put repeatedly

In this sentence, there is a single completed action, so положил is the natural choice.

Yes. In the past tense, Russian verbs show gender in the singular.

• положил = masculine
• положила = feminine
• положило = neuter
• положили = plural

So я положил suggests that the speaker is male.
If the speaker were female, it would be я положила пакетик чая в кружку.

Because чая is in the genitive case.

Russian often uses the genitive after a noun that means a container, amount, or unit of something. So:

• пакетик чая = a packet of tea / a teabag
• чашка чая = a cup of tea
• бутылка воды = a bottle of water

Here:

• пакетик is the main noun
• чая tells you what is inside the packet

So the structure is basically packet of tea.

It is a normal and very natural way to say teabag.

Literally, it means little packet of tea, and that is exactly what a teabag is.

You may also see:

• чайный пакетик = tea packet / teabag

Both are understandable and common, but пакетик чая sounds very natural in everyday speech.

Also, пакетик contains the diminutive suffix -ик, but in this word it does not necessarily sound especially cute. It is just the standard word for a small packet like a teabag.

It is the accusative singular, because it is the direct object of положил.

The speaker did what?
He put пакетик чая.

For an inanimate masculine noun, the accusative singular looks the same as the nominative singular:

• nominative: пакетик
• accusative: пакетик

So the form does not change, even though the case function does.

Because Russian uses:

• в + accusative for motion into
• в + prepositional for location inside

Here, the teabag is being moved into the mug, so we use в кружку.

Compare:

• Я положил пакетик в кружку. = I put the teabag into the mug.
• Пакетик в кружке. = The teabag is in the mug.

This is a very important Russian pattern.

Both can be translated as cup, but they are not exactly the same.

• кружка = mug
• чашка = cup

A кружка is usually:

• larger
• sturdier
• more like a mug

A чашка is usually:

• smaller
• more like a teacup or coffee cup

So in this sentence, кружка suggests a mug rather than a delicate cup.

Yes. Russian word order is more flexible than English word order.

The sentence Перед сном я положил пакетик чая в кружку is natural and clear. It starts with the time expression Перед сном, which sets the scene first: Before bed...

Other possible orders include:

• Я перед сном положил пакетик чая в кружку.
• Пакетик чая я положил в кружку перед сном.

These versions are grammatically possible, but they shift emphasis. The original version is a neutral, natural way to begin with when the action happened.

Because Russian does not have articles like English a/an and the.

So:

• пакетик чая could mean a teabag or the teabag
• в кружку could mean into a mug or into the mug

The exact meaning depends on context.

That is completely normal in Russian, and learners gradually get used to understanding definiteness from the situation rather than from a separate word.