Мне трудно угадывать, что человек чувствует.

Because Russian often uses an impersonal construction for ideas like It is hard for me...

• мне is the dative case of я
• literally, Мне трудно... is something like To me, it is difficult...

So Russian does not say Я трудно угадывать. That would be ungrammatical.

This pattern is very common:

• Мне трудно понять. = It’s hard for me to understand.
• Ему легко говорить по-русски. = It’s easy for him to speak Russian.
• Нам сложно выбрать. = It’s hard for us to choose.

Трудно means difficult / hard, but in this sentence it is not acting like a normal adjective modifying a noun.

It is a predicative word (sometimes called a category of state word), used in sentences like:

• Мне трудно. = It’s hard for me.
• Здесь холодно. = It’s cold here.
• Ему стыдно. = He is ashamed / It is shameful for him.

Compare:

• трудная задача = a difficult task
  Here трудная is a regular adjective.
• мне трудно = it is hard for me
  Here трудно is the predicative form.

So in your sentence, трудно means it is hard.

Russian often leaves out the dummy subject it that English needs.

English says:

• It is hard for me to guess...

Russian simply says:

• Мне трудно угадывать...

There is no need for a subject like это here. In fact, Это мне трудно угадывать would sound different and would usually add emphasis, something like This is hard for me to guess.

So the lack of it is normal in Russian.

Because after words like трудно, легко, приятно, важно, Russian often uses an infinitive to say to do something.

So:

• Мне трудно угадывать = It is hard for me to guess
• Ему легко читать = It is easy for him to read
• Нам важно знать = It is important for us to know

The infinitive expresses the action in a general way, just like English to guess.

This is an aspect question.

• угадывать = imperfective
• угадать = perfective

Here the imperfective угадывать is used because the sentence talks about a general difficulty, not one single completed guess.

So the meaning is more like:

• I find it hard to guess / read what a person is feeling in general

If you said:

• Мне трудно угадать, что человек чувствует

that would sound more like:

• It’s hard for me to guess what the person is feeling
  in a particular situation, or as a single act of figuring it out.

So:

• угадывать = general ability / repeated situations
• угадать = one specific successful guess

Both can be possible, but they are not exactly the same.

Not quite just guess in the random sense.

Угадывать can mean:

• to guess correctly
• to figure out
• to sense
• to read

In this sentence, it often has the nuance of:

• to tell
• to figure out
• to read what someone is feeling

So the sentence suggests difficulty in understanding another person’s emotions from clues, expression, tone, behavior, and so on.

It is often a little more intuitive than plain думать or решать.

Because что человек чувствует is a subordinate clause.

Russian normally separates subordinate clauses with a comma:

• Я знаю, что он дома.
• Мне интересно, что ты думаешь.
• Мне трудно угадывать, что человек чувствует.

So the comma is required here.

Here что is best understood as introducing an indirect question:

• what a person feels

So this is closer to English what than to plain that.

Compare:

• Я знаю, что он дома. = I know that he is home.
  Here что is a conjunction meaning that.
• Я знаю, что он чувствует. = I know what he feels / what he is feeling.
  Here что introduces the content of an indirect question.

In your sentence, it is the second type.

Человек is singular, but Russian often uses the singular to talk about people in general.

So:

• что человек чувствует

can mean:

• what a person feels
• what someone feels
• what people feel, in a general human sense

This is very natural in Russian. English can also do this with a person or someone.

Because Russian has no articles, человек can mean:

• a person
• the person
• man / human being

The exact meaning depends on context. Here it is best understood generically: a person.

Russian present tense is often used where English uses either:

• feels
• is feeling

So что человек чувствует can mean:

• what a person feels
• what a person is feeling

Russian does not have a separate continuous form like English is feeling. The plain present tense often covers both meanings.

Because чувствовать and чувствовать себя are related but different.

• чувствовать = to feel something
• чувствовать себя = to feel, in the sense of to feel oneself / to be feeling physically or emotionally

Compare:

• Он чувствует страх. = He feels fear.
• Я чувствую, что что-то не так. = I feel that something is wrong.
• Он плохо себя чувствует. = He feels unwell.

In your sentence, the meaning is about what emotion or inner state the person feels, so чувствует is the right choice.

Yes. Russian word order is fairly flexible, though the neutral order here is very natural:

• Мне трудно угадывать, что человек чувствует.

Other orders are possible, for example:

• Угадывать, что человек чувствует, мне трудно.
• Что человек чувствует, мне трудно угадывать.

These alternatives usually add emphasis or sound more literary or contrastive.

For a learner, the original order is the safest and most neutral.

Yes, and it would be very natural, but the meaning shifts a little.

• понять = understand
• угадывать = guess / figure out / read

So:

• Мне трудно понять, что человек чувствует
  = It is hard for me to understand what a person is feeling.

This focuses more on understanding.

• Мне трудно угадывать, что человек чувствует
  = It is hard for me to guess/read what a person is feeling.

This focuses more on figuring it out from signs.

So they are similar, but not identical.

Yes, it is grammatical and understandable.

That said, depending on context, native speakers might also say things like:

• Мне трудно понять, что человек чувствует.
• Мне трудно угадать, что человек чувствует.
• Мне трудно понять чувства человека.
• Мне трудно распознавать чужие эмоции.

Your sentence sounds natural if you want the nuance of having trouble reading what someone feels. It is especially good when the idea is about difficulty interpreting another person’s emotional state.

A fairly literal breakdown is:

• Мне = to me / for me
• трудно = difficult / hard
• угадывать = to guess / to figure out
• что человек чувствует = what a person feels / is feeling

So the structure is basically:

• To me, it is hard to guess what a person feels

That helps explain why Russian uses мне and the infinitive instead of a subject like I plus a finite verb.