Настоящие друзья поддерживают друг друга, когда им трудно.

Настоящий can mean different things depending on context.

In настоящие друзья, it means true, real, or genuine:

• настоящий друг = a true friend

In other contexts, it can mean present or current:

• в настоящее время = at the present time

So here, because it is describing friends, the natural meaning is true friends, not present friends.

Because the sentence is talking about more than one friend.

• друг = friend (singular)
• друзья = friends (plural)

The adjective has to agree with the noun:

• настоящий друг = a true friend
• настоящие друзья = true friends

Here both words are in the nominative plural, because they form the subject of the sentence.

Друзья is in the nominative plural.

It is the subject of the verb поддерживают, so nominative is the expected case:

• Кто поддерживает? — друзья
• Who supports? — friends

So:

• настоящие друзья = the subject
• поддерживают = what they do

Поддерживают is:

• 3rd person plural
• present tense
• imperfective aspect

It agrees with друзья:

• друг поддерживает = a friend supports
• друзья поддерживают = friends support

The imperfective is used because this sentence expresses a general truth / habitual idea:

• true friends support each other
• not just one single completed act, but something they do in general

Because поддерживают and поддержат differ in aspect and meaning.

• поддерживают = imperfective, used for repeated, regular, or general actions
• поддержат = perfective future, meaning will support in a specific future situation

This sentence is stating a general rule about true friends, so поддерживают is the natural choice.

Compare:

• Настоящие друзья поддерживают друг друга.
  True friends support each other.
• Настоящие друзья поддержат тебя.
  True friends will support you.

Друг друга means each other or one another.

Russian often uses this two-word expression where English uses a single reciprocal expression:

• Они любят друг друга. = They love each other.
• Они поддерживают друг друга. = They support each other.

In this sentence, друг друга is the direct object of поддерживают, so it appears in the form used for that function.

A useful thing to remember is that друг друга behaves like a fixed reciprocal expression:

• друг друга = each other
• друг другу = to each other
• друг с другом = with each other

So here, just learn поддерживать друг друга as a very common pattern.

Because Russian uses a different structure from English here.

Им трудно literally means something like:

• to them, it is difficult
• more naturally: it is hard for them

Here:

• им = to them / for them (dative case)
• трудно = hard / difficult

Russian often uses the dative case with words describing a person’s state or experience:

• мне трудно = it is hard for me
• ему легко = it is easy for him
• нам холодно = we are cold / it is cold for us

So они трудно would be ungrammatical.

Here трудно is not an adjective agreeing with a noun. It is a predicative word (often taught as an adverb-like form meaning a state).

It expresses a condition or situation:

• им трудно = it is hard for them

Compare:

• трудный день = a difficult day
  Here трудный is an adjective.
• им трудно = it is hard for them
  Here трудно describes a state.

So in this sentence, трудно does not agree with им in gender or number.

Because Russian normally omits the verb “to be” in the present tense.

In English, you say:

• when it is hard for them

In Russian, the present-tense is is usually not expressed:

• когда им трудно

A more literal gloss would be:

• when for them [it is] hard

This is completely normal Russian.

Because когда им трудно is a subordinate clause.

The main clause is:

• Настоящие друзья поддерживают друг друга
  True friends support each other

The subordinate clause is:

• когда им трудно
  when things are hard for them

Russian punctuation normally puts a comma before a subordinate clause introduced by words like когда, если, потому что, and so on.

Yes, Russian word order is fairly flexible, though some orders sound more neutral than others.

The given sentence is neutral and natural:

• Настоящие друзья поддерживают друг друга, когда им трудно.

You could also say:

• Когда им трудно, настоящие друзья поддерживают друг друга.

This puts more emphasis on the situation when things are hard.

So the meaning stays basically the same, but the focus can shift depending on word order.

It can mean both.

Поддерживать is a broad verb meaning to support, and in real usage it can refer to:

• emotional support
• moral support
• practical help
• standing by someone in a difficult time

In this sentence, because the subject is true friends and the condition is when it is hard for them, the meaning is naturally broad: true friends are there for each other in difficult times.

A helpful stress-marked version is:

Настоя́щие друзья́ подде́рживают друг дру́га, когда́ им тру́дно.

A few pronunciation notes:

• друзья is stressed on the last syllable: дру-ЗЬЯ
• трудно is stressed on тру́д-
• поддерживают has the stress on де́р

If you read it smoothly, the sentence sounds natural with a pause before the subordinate clause:

• Настоя́щие друзья́ подде́рживают друг дру́га, | когда́ им тру́дно.