Я положил зубную нить рядом с зубной щёткой.

Положить is the infinitive, meaning to put.

In the sentence, you need the past tense form: положил, which means put or placed.

• положить = to put
• положил = he/I put
• положила = she/I put
• положили = they/we/you put

Because Russian past tense agrees with gender and number, положил tells you the speaker is grammatically masculine singular. So this sentence sounds like it was said by a man.

Only the past-tense verb changes:

• Я положил зубную нить рядом с зубной щёткой. = said by a man
• Я положила зубную нить рядом с зубной щёткой. = said by a woman

Russian past tense works this way regularly:

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

Everything else in the sentence stays the same.

Because зубную нить is the direct object of the verb положил.

The verb положить takes the accusative case for the thing being placed. So зубная нить changes to the accusative:

• nominative: зубная нить
• accusative: зубную нить

A useful detail:

• the noun нить itself stays the same in nominative and accusative singular
• the adjective changes: зубная → зубную

So the case marking is especially visible on the adjective.

In Russian, рядом often works together with с to mean next to or beside.

So:

• рядом с чем-то = next to something

This is just the standard pattern to learn as a chunk:

• рядом с домом = next to the house
• рядом с машиной = next to the car
• рядом с зубной щёткой = next to the toothbrush

So it is best to memorize рядом с + instrumental.

Because after рядом с, Russian uses the instrumental case.

So зубная щётка changes to instrumental:

• nominative: зубная щётка
• instrumental: зубной щёткой

Both the adjective and noun change:

• зубная → зубной
• щётка → щёткой

This is one of the most common things learners need to get used to: after certain prepositions, Russian requires a specific case.

Because adjectives in Russian must agree with the noun in gender, number, and case.

Here the adjective зубной belongs to two different nouns in two different cases:

1. зубную нить

   • feminine singular
   • accusative
   • so the adjective becomes зубную

2. зубной щёткой

   • feminine singular
   • instrumental
   • so the adjective becomes зубной

So the stem is the same, but the ending changes to match the grammar of each phrase.

Yes. Положил is the past tense of the perfective verb положить.

Perfective is used when the action is seen as completed: the floss was placed and the action is finished.

That fits this sentence well, because it describes one completed act.

Compare:

• положил = put, placed, finished doing it
• клал = was putting, used to put, put repeatedly, or in some broader context

For a simple one-time completed action like this, положил is the natural choice.

Yes, very often.

Russian usually allows you to omit the subject pronoun when it is clear from the verb form or from context. So:

• Я положил зубную нить рядом с зубной щёткой.
• Положил зубную нить рядом с зубной щёткой.

Both can work, depending on context.

Including Я may sound a bit more explicit, contrastive, or simply clear. Omitting it is very common in natural speech.

No, Russian word order is flexible.

The sentence as given is a very normal neutral order:

• Я положил зубную нить рядом с зубной щёткой.

But other orders are possible, for example:

• Зубную нить я положил рядом с зубной щёткой.
• Рядом с зубной щёткой я положил зубную нить.

The basic meaning stays the same, but the focus changes. Russian often moves words around to highlight what is new, important, or contrasted.

For learners, the original order is a good neutral model.

The letter ё is very helpful because it always shows a stressed yo sound.

So щёткой is pronounced with stress on that syllable:

• щёткой = roughly SHCHYOT-koy

A few useful points:

• ё is always stressed
• щ is a soft sound, often approximated as a long soft sh
• the ending -ой here is pronounced roughly like -oy

So the stressed part of the whole phrase is:

• зубно́й щёткой

Also, in careful learning materials, ё is often written, but in many real texts it may be replaced by е. So you may sometimes see щеткой, even though it is still pronounced щёткой.

Yes, зубная нить is the standard and neutral term for dental floss.

Literally it is dental thread or tooth thread, which is how Russian expresses the idea.

In everyday speech, native speakers may also shorten or rephrase things depending on context, but зубная нить is the normal dictionary-style expression and is completely natural.

Yes, but the nuance is a little different.

• рядом с = right next to, beside
• возле = near, by
• около = near, around, close to

So:

• рядом с зубной щёткой suggests the floss is placed right next to the toothbrush
• возле зубной щётки or около зубной щётки can sound a bit less exact, more like near the toothbrush

Also, the grammar changes:

• рядом с
  • instrumental
• возле
  • genitive
• около
  • genitive

So you cannot just swap the preposition and keep the same noun form.

The normal stresses are:

Я положи́л зубну́ю нить ря́дом с зубно́й щёткой.

A few useful notes:

• положи́л — stress on the last syllable
• зубну́ю — stress on -ну́-
• ря́дом — stress on the first syllable
• зубно́й — stress on the last syllable
• щёткой — stress on щё

Learning stress early is very important in Russian, because it is not always predictable from spelling.