Если кто-либо заметит ошибку, скажите мне.

Кто-либо means anyone / anybody / whoever it may be.

In this sentence, если кто-либо заметит ошибку means if anyone notices a mistake.

For an English speaker, an important point is that кто-либо is a bit more formal and impersonal than кто-то.

• кто-либо = anyone, often in formal, written, or general statements
• кто-то = someone, often more neutral or conversational

So you could also hear:

• Если кто-то заметит ошибку, скажите мне.

That sounds a little more natural in everyday speech, while кто-либо sounds more formal.

Заметит is the perfective future form of заметить.

This is used because the sentence talks about a single possible future event:

• If anyone notices a mistake...

In Russian, perfective verbs often express a completed or one-time action. Here, noticing the mistake is treated as one event.

Compare:

• заметит = will notice / happens to notice
• замечает = notices / is noticing / tends to notice

In an если clause about a future possibility, Russian commonly uses the perfective future:

• Если он придёт, я скажу ему.
• Если кто-либо заметит ошибку, скажите мне.

So заметит is exactly what you would expect here.

Yes, it refers to the future, even though it may look like a present form to a learner.

That is because заметить is a perfective verb. Perfective verbs do not have a true present tense. Their so-called present-looking forms actually refer to the future.

So:

• заметить → замечу, заметишь, заметит... = future
• замечать → замечаю, замечаешь, замечает... = true present / habitual

Therefore:

• Он заметит ошибку = He will notice the mistake
• Он замечает ошибки = He notices mistakes

Because it is the direct object of the verb заметить.

The verb заметить means to notice, and the thing being noticed goes into the accusative case.

Base form:

• ошибка = mistake / error (nominative)

Accusative singular:

• ошибку

So:

• заметить ошибку = to notice a mistake

This is a very common feminine noun pattern:

• книга → книгу
• машина → машину
• ошибка → ошибку

Скажите is the imperative form used for:

1. you plural
2. polite singular you

So here it means tell me addressed either to several people or to one person politely.

Compare:

• скажи мне = tell me (singular, informal)
• скажите мне = tell me (plural or polite singular)

Since the sentence sounds formal already because of кто-либо, скажите fits that tone well.

Because сказать takes an indirect object in the dative case when you say tell someone.

So:

• сказать кому? = to tell whom?
• мне is the dative form of я

Examples:

• Скажите мне. = Tell me.
• Я скажу ему. = I will tell him.
• Она сказала нам. = She told us.

Меня is not correct here because меня is usually accusative or genitive, not dative.

No, Russian word order is fairly flexible.

The given sentence:

• Если кто-либо заметит ошибку, скажите мне.

is completely natural. But you could also say:

• Скажите мне, если кто-либо заметит ошибку.

Both mean essentially the same thing: If anyone notices a mistake, tell me.

The choice depends on emphasis and style. Putting the если clause first is very common when setting up a condition.

Here если clearly means if.

The sentence describes a possible condition:

• If anyone notices a mistake, tell me.

Russian если is used for real conditions, just like English if.

If you wanted when, Russian would normally use когда instead:

• Когда кто-либо заметит ошибку, скажите мне.

But that version sounds less natural in this context, because the speaker is not assuming the event will definitely happen. They are simply allowing for the possibility.

It is correct and natural, but it sounds somewhat formal.

A more everyday version would often be:

• Если кто-то заметит ошибку, скажите мне.
• Если кто-то заметит ошибку, сообщите мне.
• Если заметите ошибку, скажите мне.

These differ slightly in tone:

• кто-либо = more formal, more written
• кто-то = more conversational
• сообщите мне = more formal than скажите мне
• Если заметите ошибку... = very natural if you are addressing the people directly

So the original sentence is good, especially in formal instructions or announcements.

The verb pair is:

• замечать = imperfective
• заметить = perfective

This matters because Russian aspect shows how the action is viewed.

Here, заметить presents the action as a single completed event: someone notices the mistake.

Compare:

• Если кто-либо замечает ошибки...
  This would sound more like if someone tends to notice errors or is noticing errors, which is not the intended meaning here.

• Если кто-либо заметит ошибку...
  This means if someone notices a mistake in a specific instance.

So the perfective aspect is the natural choice.

Yes. If you wanted to say mistakes rather than a mistake, you could say:

• Если кто-либо заметит ошибки, скажите мне.

Here:

• ошибки is the accusative plural form

Inanimate plural nouns usually have the same form in nominative and accusative, so ошибки works for both.

The singular ошибку suggests one error, but in practice it can also function a bit like English a mistake in a general sense.