Где бы ни был мой друг, он всё равно пишет мне вечером.

Где бы ни is a set pattern meaning wherever / no matter where.

In this sentence:

• Где = where
• бы ... ни together create the idea of no matter where

So Где бы ни был мой друг means something like:

• Wherever my friend is
• No matter where my friend may be

This is a very common Russian structure:

• кто бы ни... = whoever
• что бы ни... = whatever
• как бы ни... = no matter how
• когда бы ни... = whenever

It often introduces a clause that does not name a specific place/person/time, but says it does not matter which one.

Because in this kind of expression, бы and ни work together as a grammatical pattern.

In concessive clauses like wherever, whoever, whatever, Russian often uses:

• a question word (где, кто, что, когда, etc.)
• plus бы
• plus ни
• plus the verb

So:

• Где бы ни был...
• Кто бы ни пришёл...
• Что бы ни случилось...

This pattern means it does not matter where/who/what.

Important: this ни is not the same as ordinary negation. It is part of a fixed construction. You usually should learn где бы ни, кто бы ни, etc. as whole patterns.

Because ни is used in this special concessive pattern, while не is the usual particle of negation.

Compare:

• Он не дома. = He is not at home.
  Here не simply negates.

But in:

• Где бы ни был мой друг...

ни does not mean simple not. It helps form the meaning wherever / no matter where.

So for English speakers, it is best not to translate ни word-for-word here. Just recognize the whole structure:

• где бы ни был = wherever he is

Russian usually does not use есть as a present-tense equivalent of is in sentences like this.

In modern Russian:

• Он дома. = He is at home.
• Мой друг в Москве. = My friend is in Moscow.

There is normally no present-tense verb to be.

So why был here?

Because in the construction где бы ни был, Russian uses the past-form shape with бы to express a kind of non-specific, hypothetical, or dependent meaning. It does not mean literal past time here.

So:

• Где бы ни был мой друг does not mean wherever my friend was
• it means wherever my friend may be / is

This is one of those places where Russian uses the past-form-looking verb with бы in a non-past meaning.

Yes, both are possible:

• Где бы ни был мой друг
• Где бы мой друг ни был

Both mean the same thing: Wherever my friend is.

Russian word order is more flexible than English word order. The difference is usually about style, rhythm, or emphasis, not core meaning.

In your sentence, Где бы ни был мой друг sounds natural and smooth. Putting мой друг after был is very common in this type of clause.

So as a learner, you should recognize both patterns.

Всё равно means all the same, anyway, still, or regardless.

In this sentence:

• Где бы ни был мой друг, он всё равно пишет мне вечером.

the idea is:

• No matter where my friend is, he still writes to me in the evening.

It emphasizes that the second action does not change despite the first circumstance.

Very common uses:

• Мне всё равно. = It doesn’t matter to me / I don’t care.
• Он устал, но всё равно работает. = He is tired, but he works anyway.

Here it has the sense of despite that, still.

Because the verb писать when it means to write to someone takes the dative case for the person receiving the writing.

So:

• мне = to me (dative of я)
• тебе = to you
• ему = to him
• нам = to us

Examples:

• Она пишет мне каждый день. = She writes to me every day.
• Я пишу другу. = I write to my friend.

If you want to mention the thing being written, that is separate:

• Он пишет мне письмо. = He is writing me a letter / He writes a letter to me.

So мне is used because it means to me, not as a direct object.

Вечером is the instrumental case, and Russian often uses the instrumental without a preposition to mean in the morning / in the afternoon / in the evening.

So:

• утром = in the morning
• днём = in the daytime / in the afternoon
• вечером = in the evening
• ночью = at night

Therefore:

• он пишет мне вечером = he writes to me in the evening

This is a standard time expression, so it is best learned as a chunk.

Here пишет is best understood as habitual or regular present.

So the meaning is not necessarily:

• he is writing to me right now in the evening

but rather:

• he writes to me in the evening
• he keeps writing to me every evening / in the evenings

Russian present tense can express both:

1. an action happening now
2. a repeated/habitual action

In this sentence, because of вечером and the overall meaning, the habitual reading is the natural one.

Because the sentence describes a repeated, ongoing habit, not a single completed act.

• писать = imperfective, to write
• написать = perfective, to write / finish writing

Here the idea is that no matter where he is, he writes to me in the evening as a regular thing. That calls for the imperfective:

• он пишет мне вечером

If you used a perfective form, it would sound more like a single completed event or a future completion, which does not fit the general habitual meaning.

Grammatically, yes: друг is masculine, and был also agrees with a masculine subject.

So this sentence refers to a male friend.

If the friend were female, you would say:

• Где бы ни была моя подруга, она всё равно пишет мне вечером.

Changes:

• друг → подруга
• мой → моя
• был → была
• probably он → она

Because Где бы ни был мой друг is a subordinate clause, and it is separated from the main clause by a comma.

Structure:

• subordinate clause: Где бы ни был мой друг
• main clause: он всё равно пишет мне вечером

This is the same basic idea as in English:

• Wherever my friend is, he still writes to me in the evening.

Russian normally separates such clauses with a comma.

Yes. For understanding the grammar, may be is actually a very helpful English approximation.

• Где бы ни был мой друг ≈ Wherever my friend may be

That does not mean the Russian sentence is especially formal or old-fashioned. In normal English, the most natural translation is often simply:

• Wherever my friend is, he still writes to me in the evening.

But for grammar study, may be helps show that the clause is not talking about a specific known location. It has that no matter where he happens to be feeling.