Новая грамматическая тема: конструкция «друг другу» означает «to each other / each other».

Russian usually expresses each other with a two-part construction based on друг. Historically, it comes from the idea of one another.

So друг другу is not random repetition. It is a fixed reciprocal pattern meaning that the action goes both ways between people.

For example:

• Они помогают друг другу. = They help each other.
• Мы часто пишем друг другу. = We often write to each other.

Both words are part of the structure.

Yes, it is the same word historically: друг can mean friend.

But in the expression друг другу, you should not translate it literally as friend to friend. In this construction, it functions as part of a grammatical pattern meaning each other.

So:

• друг by itself can mean friend
• друг другу is a fixed reciprocal expression meaning to each other

In друг другу, the second word другу is in the dative case, which often corresponds to to someone.

That is why друг другу is used when the verb requires the dative:

• Они помогают друг другу. = They help each other.
• Мы звоним друг другу. = We call each other.
• Она всё объяснила им, и они всё объяснили друг другу. = She explained everything to them, and they explained everything to each other.

The first друг stays in this fixed form, while the second part changes according to case.

Use the form that matches the case required by the verb or preposition.

A few very common forms are:

• друг друга — usually each other in the genitive/accusative

  • Они любят друг друга. = They love each other.
  • Мы видим друг друга. = We see each other.

• друг другу — to each other in the dative

  • Они помогают друг другу. = They help each other.
  • Мы пишем друг другу. = We write to each other.

• друг другом — with each other / by each other in the instrumental

  • Они довольны друг другом. = They are pleased with each other.

• друг о друге / друг друге with a preposition — depends on the preposition

  • Они думают друг о друге. = They think about each other.

So друг другу is just one case form of the larger друг + друг reciprocal pattern.

In the basic pattern, the second word changes to show the needed case, while the first word usually stays друг.

Compare:

• друг друга
• друг другу
• друг другом
• друг о друге

So learners usually focus on changing the second part first.

That said, in some contexts, especially with prepositions or more complex phrasing, you may see variation, but for normal learning purposes it is best to memorize the standard set of forms above.

It can be used for two or more people.

Examples:

• Мы помогаем друг другу. = We help each other.
  This can mean two people or several people.

• Студенты часто помогают друг другу. = The students often help each other.

So it does not mean strictly one person to one other person. It can refer to mutual action inside a group.

Normally, no. The idea of each other requires at least two participants, so the subject is usually plural or collective.

Common subjects are:

• мы
• вы
• они
• plural nouns like друзья, студенты, родители

Examples:

• Мы доверяем друг другу.
• Они не мешают друг другу.

A singular subject like он помогает друг другу is not correct, because one person cannot do something to each other alone.

The expression is fairly flexible, but it usually comes after the verb or near it.

Examples:

• Они помогают друг другу.
• Мы часто звоним друг другу.
• Они друг другу не доверяют.

Russian word order is more flexible than English, so speakers may move друг другу for emphasis. But as a learner, the safest pattern is:

subject + verb + друг другу

For example:

• Мы пишем друг другу.

Yes, but then the form changes according to the preposition.

For example:

• друг о друге = about each other

  • Они часто говорят друг о друге.

• друг с другом = with each other

  • Они хорошо ладят друг с другом.

• друг у друга = at each other’s place / from each other / in each other, depending on context

  • Мы были друг у друга в гостях. = We visited each other.

So you should not always use друг другу. Use the form that matches the grammar of the sentence.

It is especially common with verbs that take the dative case.

Very common examples:

• помогать — to help
  Они помогают друг другу.

• звонить — to call
  Мы звоним друг другу.

• писать — to write
  Они пишут друг другу.

• доверять — to trust
  Вы доверяете друг другу?

• мешать — to interfere with / bother
  Они не мешают друг другу.

A good learning strategy is to memorize друг другу together with dative verbs.

It is completely normal and very common in everyday Russian. It is the standard way to say to each other when the verb requires the dative.

You will hear it in ordinary speech all the time:

• Они уже давно знают друг друга.
• Мы редко звоним друг другу.
• Надо помогать друг другу.

So this is not a literary or unusual expression. It is basic, useful everyday grammar.