Какой бы трудной ни казалась ситуация сначала, её легче пережить, если люди умеют выслушать друг друга.

Because with казаться (to seem), the description is normally put in the instrumental case:

• ситуация казалась трудной = the situation seemed difficult

So both words that describe the situation take instrumental feminine singular:

• какой
• трудной

Here какой is the feminine instrumental form of какой. It looks unusual because the feminine forms of this word overlap in several cases.

This is a very common Russian concessive pattern. It means something like:

• however ...
• no matter how ...

So:

• Какой бы трудной ни казалась ситуация...
  means However difficult the situation may seem...

The general pattern is:

• кто бы ни... = whoever
• что бы ни... = whatever
• где бы ни... = wherever
• как бы ни... = however / no matter how

No, this is not a negative clause.

In this structure, ни is part of the fixed concessive pattern ... бы ... ни .... It does not mean that the situation did not seem difficult. Instead, it helps create the sense of no matter how.

So ни here is grammatical and idiomatic, not a separate negation.

Because the subject is ситуация, which is feminine singular.

In the past tense, Russian verbs agree with the subject in gender and number:

• он казался
• она казалась
• оно казалось
• они казались

So:

• ситуация казалась

The ending -сь is there because the verb is казаться.

Russian word order is much freer than English word order.

In this sentence, the speaker puts the quality first:

• какой бы трудной...

and only then names the noun:

• ...ни казалась ситуация

That is very natural in Russian, especially in this concessive pattern. It gives emphasis to how difficult the situation seems.

So the order is not random; it is a normal stylistic choice.

Сначала is an adverb meaning:

• at first
• initially
• in the beginning

Here it modifies казалась:

• ситуация сначала казалась трудной
• the situation seemed difficult at first

So it tells you that this is how the situation appeared in the beginning, not necessarily later.

Because in the present tense, Russian normally does not use a separate word for is / am / are.

So:

• её легче пережить

literally looks like:

• it easier to get through it

but naturally means:

• it is easier to get through it

This is a very normal Russian structure:
comparative + infinitive

For example:

• мне легче понять = it is easier for me to understand
• это трудно сделать = it is hard to do this

Because пережить is perfective, and here the idea is to get through a specific situation as a whole.

• пережить ситуацию = get through the situation
• переживать would focus more on the process, repeated action, or general experience

So её легче пережить means it is easier to come through that situation successfully.

Её refers back to ситуация.

It is in the accusative case because it is the direct object of пережить:

• пережить кого? что?
• её

So:

• ситуация = nominative
• её = accusative, referring to that same situation

Уметь + infinitive usually means to know how to do something, or to be able through skill, habit, or developed ability.

So:

• люди умеют выслушать друг друга

suggests that people have the human skill or emotional ability to listen properly to one another.

If you used могут, it would sound more like they can / are able to in a situational sense.
Here умеют is more about interpersonal competence.

Слушать means to listen.

Выслушать means something stronger: to listen to someone fully, to hear someone out.

So this sentence is not just about hearing sounds or paying some attention. It is about giving the other person a real chance to speak and being willing to listen to them properly.

Also, выслушать is perfective, which fits the idea of listening someone out to completion.

Because друг друга is the form used here for each other in the case required by the verb.

The verb выслушать takes the accusative:

• выслушать кого? = to listen to whom?

So the reciprocal expression must also be in that case:

• выслушать друг друга = listen to each other / hear each other out

Compare:

• помогать друг другу = help each other (dative)
• говорить друг о друге = talk about each other

Because the sentence contains two subordinate clauses.

Structure:

• Какой бы трудной ни казалась ситуация сначала,
  concessive clause

• её легче пережить,
  main clause

• если люди умеют выслушать друг друга.
  conditional clause

So the commas separate:

1. the opening concessive clause from the main clause
2. the main clause from the если clause

This punctuation is standard in Russian.