Если Вы пишете письмо начальнице, лучше не использовать сокращения.

Capitalized Вы is a polite, formal way to write you in Russian, especially in letters, emails, and other respectful communication.

• Вы = formal/polite you, or plural you
• ты = informal singular you

In this sentence, the capital letter suggests the sentence is talking about addressing one person politely, not a group. In modern Russian, lowercase вы is also very common even in polite use, but uppercase Вы adds extra formality.

Because начальнице is in the dative case.

The noun начальница means female boss / female manager / female superior. After the idea write a letter to someone, Russian uses the dative for the person receiving it:

• писать письмо кому? = to write a letter to whom?
• начальнице = to the female boss

So:

• начальница = dictionary form, nominative
• начальнице = dative singular

Russian often expresses to someone with the dative case alone, without a preposition.

So:

• писать начальнице = to write to the female boss
• писать письмо начальнице = to write a letter to the female boss

This is very normal Russian grammar. English needs to, but Russian does not here.

Yes, пишете is the present tense form of писать for вы.

In Russian, the present tense is often used in general conditions:

• Если Вы пишете письмо начальнице...
  = If you are writing / if you write a letter to your boss...

This does not have to mean the action is happening right now. It can describe a general situation or repeated situation, much like English if you write.

Лучше не использовать is a softer, more natural recommendation:

• лучше не использовать = it’s better not to use
• не должны использовать = you must not / should not use

So лучше не использовать sounds like advice about good style or etiquette, not a strict rule. It fits the context well: writing to your boss calls for a more formal style, so abbreviations are better avoided.

Because the construction лучше + infinitive is very common in Russian.

Examples:

• Лучше подождать. = It’s better to wait.
• Лучше не спорить. = It’s better not to argue.
• Лучше не использовать сокращения. = It’s better not to use abbreviations.

This is an impersonal structure. Russian does not need to say you again. The subject is understood from the context.

Сокращения is the plural of сокращение (abbreviation, shortening).

Here it is in the accusative plural, because it is the direct object of использовать:

• использовать что? = to use what?
• сокращения

For inanimate plural nouns, the accusative usually looks the same as the nominative. So:

• nominative plural: сокращения
• accusative plural: сокращения

That is why the form does not change.

Because Если Вы пишете письмо начальнице is a subordinate clause introduced by если (if), and Russian normally separates that clause with a comma.

So the structure is:

• Если ... , ...
• If ... , ...

This is standard punctuation in Russian.

Not exactly.

• если = if
• когда = when

In this sentence, если gives a conditional meaning: in the situation where you are writing to your boss, this advice applies.

If you used когда, it would sound more like when(ever) you write a letter to your boss, which can also be possible in some contexts, but it changes the nuance from condition to time/habit.

Because начальница specifically refers to a female boss. The sentence is talking about writing to a woman.

Compare:

• начальнику = to a male boss or sometimes to a boss in a gender-neutral/traditional masculine form
• начальнице = to a female boss

So this sentence explicitly tells you the boss is female.

Yes. Russian often omits subject pronouns when they are clear from the verb form.

So you could say:

• Если пишете письмо начальнице, лучше не использовать сокращения.

This still sounds natural. Including Вы, especially capitalized Вы, makes the sentence more explicit and polite/formal.

It is not a single fixed idiom, but it is a very normal combination of words.

• письмо = letter
• письмо начальнице = a letter to the female boss

The pattern is very common:

• письмо другу = a letter to a friend
• письмо маме = a letter to Mom
• письмо преподавателю = a letter to the teacher

So this is a regular and useful structure to learn.