Эти капли помогают, но их нельзя использовать слишком часто.

Because капли is plural, and the demonstrative adjective has to agree with it.

• этот = this (masculine singular)
• эта = this (feminine singular)
• это = this (neuter singular)
• эти = these (plural)

So эти капли means these drops.

The singular of капли is капля, which is feminine:

• эта капля = this drop

But in your sentence the noun is plural, so we use эти.

Капли literally means drops.

In everyday Russian, капли often refers to medicinal drops, for example:

• eye drops
• nose drops
• ear drops

So in this sentence, it most likely means something like these drops in a medical sense, not just random drops of liquid.

Because the subject is капли, which is plural.

Russian verbs in the present tense agree with the subject:

• капля помогает = the drop helps
• капли помогают = the drops help

So Эти капли помогают literally means These drops help.

Because их is the form used for a direct object, while они is the subject form.

Compare:

• они помогают = they help
  
  • они is the subject
• использовать их = to use them
  
  • их is the object

In the sentence:

• Эти капли помогают — капли are the subject
• но их нельзя использовать... — now them is the object of использовать

So English also does the same thing:

• they help
• you cannot use them

not you cannot use they

Нельзя is an impersonal word that often means:

• it is not allowed
• one must not
• it is impossible / not possible

In this sentence, the natural meaning is:

• их нельзя использовать слишком часто
• they must not be used too often
• or you shouldn’t use them too often

A very common Russian pattern is:

• нельзя + infinitive

For example:

• нельзя курить = smoking is not allowed / you must not smoke
• нельзя трогать = do not touch / must not touch
• нельзя использовать = must not use / cannot be used

Because it comes after нельзя.

Russian often uses this structure:

• нельзя + infinitive

So:

• нельзя использовать = one must not use / it is not allowed to use

There is no personal subject like я, ты, or они here. The sentence is impersonal. Russian often expresses general rules or recommendations this way.

Because after нельзя, Russian normally uses the infinitive, not a finite verb form.

So:

• их нельзя использовать = they cannot be used / one must not use them

If you said используют, that would mean they use or are using, which would not fit this structure.

So the grammar is:

• нельзя + infinitive
• not нельзя + present-tense verb

Использовать is the imperfective verb to use. That fits well here because the sentence is talking about general repeated use, not one completed action.

• их нельзя использовать слишком часто = they should not be used too often

This is about habit/frequency, so imperfective is natural.

A perfective verb would suggest a more specific or completed act of using something, which is not the point here.

Very often, when Russian talks about:

• general rules
• habits
• repeated actions
• frequency

the imperfective is preferred.

Слишком часто means too often.

• слишком = too, excessively
• часто = often

So together:

• слишком часто = too often

This word order is normal because слишком modifies часто directly, just like English too often.

So:

• использовать слишком часто = to use too often

Yes, absolutely.

A very natural alternative is:

• Эти капли помогают, но ими нельзя пользоваться слишком часто.

This uses the verb пользоваться = to use, which takes the instrumental case:

• они → ими

So:

• использовать их = use them
• пользоваться ими = use them

Both are correct.
Использовать can sound a bit more neutral or formal, while пользоваться is also very common in everyday speech.

Yes, this is something that confuses many learners.

Их can mean:

• them
• their

These are the same forms in Russian, and the meaning depends on context.

Examples:

• Я вижу их. = I see them.
• Это их дом. = This is their house.

In your sentence:

• их нельзя использовать

the word их is clearly the object of использовать, so it means them, not their.

Because Russian avoids repeating the same noun when it is already clear.

First, the noun is named:

• Эти капли помогают = These drops help

Then, instead of repeating капли, Russian uses the pronoun:

• но их нельзя использовать слишком часто = but they/them must not be used too often

This is completely natural and works the same way in English:

• These drops help, but you can’t use the drops too often
• more natural: These drops help, but you can’t use them too often

Yes. In translation, нельзя often overlaps with both ideas.

Depending on context, it can mean:

• cannot
• must not
• should not
• it is not allowed to

In this sentence, the practical meaning is closer to:

• must not be used too often
• should not be used too often

because it sounds like a medical warning or recommendation, not physical impossibility.

So the English translation may vary slightly, but the Russian structure stays the same.