Если у тебя есть какие-либо вопросы или нужно что-либо уточнить, напиши мне вечером.

Russian usually expresses possession with the pattern:

• у + person/object in the genitive + есть
• literally: by someone there is

So:

• У тебя есть вопросы = You have questions
• literally: At you there are questions

Russian does have the verb иметь (to have), but in everyday speech it is much less common than in English and often sounds formal, bookish, or abstract. In normal conversation, у тебя есть is the natural choice.

After у, Russian uses the genitive case.

So:

• ты = you
• тебя = genitive/accusative form of ты

In the possession structure у тебя есть..., тебя is genitive because у requires it.

Examples:

• У меня есть время = I have time
• У него есть идея = He has an idea
• У тебя есть вопросы = You have questions

Какие-либо means any, some, or any kind of, depending on context.

In this sentence:

• какие-либо вопросы = any questions

This word is a bit more formal or written-sounding than the conversational alternatives:

• какие-то вопросы = some questions
• какие-нибудь вопросы = any questions / some questions

So the sentence is perfectly correct, but in everyday speech many Russians would more naturally say:

• Если у тебя есть какие-то вопросы...

They are built the same way, but they modify different kinds of words.

• какие-либо goes with a noun
  • какие-либо вопросы = any questions
• что-либо stands alone and means anything / something
  • уточнить что-либо = to clarify something / anything

So in this sentence:

• какие-либо вопросы describes the noun вопросы
• что-либо is the object of уточнить

Russian often uses impersonal constructions, where the person is not stated directly.

• нужно уточнить = it is necessary to clarify / something needs to be clarified
• In natural English here: you need to clarify something

So the full sentence does not repeat you, but it is understood from context. Russian often avoids repeating subjects when they are obvious.

You could also say:

• если тебе нужно что-либо уточнить

That version is more explicit: if you need to clarify something.

The original version is slightly more neutral and compact.

Because it follows нужно.

The pattern is:

• нужно + infinitive = need to + verb / it is necessary to + verb

Examples:

• нужно идти = need to go
• нужно позвонить = need to call
• нужно уточнить = need to clarify

So уточнить stays in the infinitive because it depends on нужно.

Напиши is the perfective imperative of написать.

Russian often uses:

• perfective imperative for a single completed action
• imperfective imperative for repeated action, process, or stronger command in some contexts

Here, the speaker means:

• write to me once this evening
• a complete action

So напиши is the natural choice.

Compare:

• напиши мне вечером = write to me this evening
• пиши мне чаще = write to me more often

Мне is the dative case of я.

The verb написать / писать often takes the person receiving the message in the dative:

• написать кому? = write to whom?

So:

• мне = to me

Examples:

• Напиши мне = Write to me
• Я написал другу = I wrote to a friend

Вечером is the instrumental case, and Russian commonly uses the instrumental to express time periods such as:

• утром = in the morning
• днём = during the day
• вечером = in the evening
• ночью = at night

So:

• напиши мне вечером = write to me in the evening

This is just a standard time expression in Russian.

Because the sentence begins with a subordinate clause introduced by если (if).

Structure:

• Если ... , напиши мне вечером.
• If ... , write to me in the evening.

Russian puts a comma between the if-clause and the main clause, just as English often does.

It is mostly informal, because it uses:

• у тебя = you singular informal
• напиши = imperative for you singular informal

However, какие-либо and что-либо sound a bit more formal or written than everyday conversation. So the sentence has a slight stylistic mix:

• grammar/pronouns: informal
• indefinite words: somewhat formal/bookish

A more casual everyday version would be:

• Если у тебя есть какие-то вопросы или нужно что-то уточнить, напиши мне вечером.

Yes. Russian word order is flexible, though the basic meaning stays the same.

Original:

• Если у тебя есть какие-либо вопросы или нужно что-либо уточнить, напиши мне вечером.

Possible variations:

• Если у тебя есть какие-либо вопросы или нужно что-либо уточнить, вечером напиши мне.
• Напиши мне вечером, если у тебя есть какие-либо вопросы или нужно что-либо уточнить.

These all mean roughly the same thing, but the emphasis changes slightly.

• putting вечером earlier emphasizes when
• putting напиши мне вечером first makes the request more prominent

The original version sounds natural and balanced.