Она положила лак для ногтей в косметичку.

Положить is the dictionary form, meaning to put / to place.

Положила is the past tense form, and it agrees with a feminine singular subject:

• он положил = he put
• она положила = she put
• оно положило = it put
• они положили = they put

So because the subject is она (she), the verb becomes положила.

In the Russian past tense, verbs usually show the gender of the subject in the singular.

For положить:

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

Since она is feminine, the verb takes the feminine ending -а: положила.

Косметичку is in the accusative case.

That is because it is the object after motion into something with в:

• в косметичку = into the makeup bag / cosmetic bag

A useful contrast:

• в косметичке = in the makeup bag (location, prepositional case)
• в косметичку = into the makeup bag (direction/result, accusative case)

Here the idea is that she put something into the bag, so Russian uses the accusative: в косметичку.

Because the sentence expresses destination/result, not just location.

Russian often uses:

• в + accusative for movement into
• в + prepositional for being in

Compare:

• Она положила лак в косметичку. = She put the nail polish into the makeup bag.
• Лак был в косметичке. = The nail polish was in the makeup bag.

So косметичку is correct because the action causes the object to end up inside the bag.

It is changed in the sense that it is in the accusative case, but for many inanimate masculine nouns, the accusative singular looks exactly like the nominative singular.

So:

• nominative: лак = nail polish / varnish
• accusative: лак

Because лак is an inanimate masculine noun, the form stays the same in the singular.

Для means for, and it requires the genitive case.

So:

• ногти = nails
• genitive plural: ногтей

That gives:

• лак для ногтей = nail polish
• literally: polish for nails

This noun + для + genitive pattern is very common in Russian:

• щётка для волос = hairbrush
• чашка для чая = cup for tea
• крем для рук = hand cream

Because the preposition для always takes the genitive case.

The base word is:

• ногти = nails

After для, it must become genitive plural:

• для ногтей = for nails

So ногтей is required by the grammar of для.

It is a normal Russian way to say nail polish, but grammatically it is a phrase, not a single word.

Literally it is:

• лак = lacquer / polish
• для ногтей = for nails

Together: лак для ногтей.

Russian often expresses ideas this way instead of using a compact compound noun like English does.

These verbs can all relate to putting something somewhere, but they are used differently.

• класть = to put, to lay down (imperfective)
• положить = to put, to place (perfective)
• ставить = to set/stand something upright

In this sentence, положила is natural because it describes a completed action: she put it into the bag.

A rough comparison:

• Она клала лак в косметичку. = She was putting / used to put the nail polish into the bag.
• Она положила лак в косметичку. = She put the nail polish into the bag.
• Она поставила бутылочку в косметичку. = She set the little bottle into the bag upright.

So положила is the best general choice here.

Yes, Russian often can omit the subject pronoun when it is clear from context.

So both of these are possible:

• Она положила лак для ногтей в косметичку.
• Положила лак для ногтей в косметичку.

Including она can add clarity, contrast, or emphasis. For example, if you are distinguishing her from someone else, keeping она makes sense.

Yes. Russian word order is more flexible than English, though different orders can sound more natural in different contexts.

Neutral order here:

• Она положила лак для ногтей в косметичку.

Other possible orders:

• Она в косметичку положила лак для ногтей.
• Лак для ногтей она положила в косметичку.
• В косметичку она положила лак для ногтей.

These usually change the focus or emphasis, not the core meaning.

For a learner, the original sentence is a very natural neutral version.

They are related, but they mean different things:

• косметика = cosmetics / makeup
• косметичка = a makeup bag, cosmetic case, toiletry bag

So in this sentence:

• в косметичку = into the makeup bag

The ending -ичка here is part of the noun; it is not just the word косметика with a random ending added.

The main stress is:

• Она́
• положи́ла
• лак
• для́
• ногте́й
• в
• космети́чку

So the sentence is pronounced roughly:

ана́ палажи́ла лак для́ нагте́й ф касмети́чку

A few notes:

• unstressed о is often pronounced closer to a
• в may sound like f before a voiceless consonant

No. Лак can mean different kinds of lacquer, varnish, or polish, depending on context.

For example:

• лак для ногтей = nail polish
• лак для волос = hairspray
• лак для дерева = wood varnish

So лак by itself is broader, and the phrase after it tells you what kind it is.