Мне трудно следить за сюжетом, если фильм слишком сложный.

Because Russian uses a very common pattern here: dative + predicative word + infinitive.

So Мне трудно следить... literally works like For me, it is difficult to follow...

The person experiencing the feeling or difficulty goes in the dative case, which is why you get мне rather than я.

If you wanted to use я, you would need a different structure, for example: Я с трудом слежу за сюжетом.

Russian often does not use a dummy subject like English it in this kind of sentence.

English says: It is hard for me to follow the plot.

Russian simply says: Мне трудно следить за сюжетом.

This is called an impersonal construction. The sentence is complete without any subject like это.

Here трудно is not an adjective agreeing with a noun. It is a predicative word or word of state, used in impersonal sentences.

It expresses a condition or evaluation:

• трудно = difficult
• легко = easy
• интересно = interesting
• нужно = necessary

So Мне трудно следить... means something like Following... is difficult for me or It is difficult for me to follow...

Because следить normally takes the preposition за when it means to follow, to keep track of, or to monitor.

So this is a set pattern: следить за + instrumental

Examples:

• следить за детьми = to watch the children
• следить за новостями = to follow the news
• следить за сюжетом = to follow the plot

So за here is not something you should translate word-for-word as behind. It is just part of the verb’s normal grammar.

Because after следить за, the noun goes in the instrumental case.

The base form is сюжет.
Its instrumental singular form is сюжетом.

So:

• сюжет = nominative
• сюжетом = instrumental

This ending is required by the verb-preposition combination следить за.

Both are possible, but сложный is much more natural in everyday speech here.

Russian can use either:

• the full adjective in the predicate: фильм сложный
• the short adjective: фильм сложен

In modern conversational Russian, the full form is usually more common in sentences like this. The short form can sound more formal, literary, or stylistically marked.

So фильм слишком сложный is the most natural choice for ordinary spoken Russian.

In the present tense, Russian usually omits the verb to be.

So English says: The film is too complicated.

Russian says: Фильм слишком сложный.

There is no present-tense есть here. That is normal Russian grammar.

Because если фильм слишком сложный is a subordinate clause, and Russian normally separates subordinate clauses with a comma.

So: Мне трудно следить за сюжетом, если фильм слишком сложный.

This is standard punctuation.

You can also reverse the order: Если фильм слишком сложный, мне трудно следить за сюжетом.

The comma is still needed.

Yes, Russian word order is flexible.

For example, this is also correct: Если фильм слишком сложный, мне трудно следить за сюжетом.

The meaning stays basically the same, but the emphasis shifts a little:

• Мне трудно следить за сюжетом... puts the main point first
• Если фильм слишком сложный... puts the condition first

However, some parts naturally stay together, especially следить за сюжетом.

Because the sentence is talking about an ongoing or general difficulty, not a single completed action.

Imperfective verbs are used for:

• general facts
• repeated situations
• ongoing processes

Here the idea is I have trouble following the plot in general, while watching a difficult film.

A perfective verb would sound wrong or would change the meaning.

Yes. Both are natural.

• трудно = difficult, hard
• сложно = difficult, complicated

In this sentence, they are very close in meaning, and both sound normal:

• Мне трудно следить за сюжетом.
• Мне сложно следить за сюжетом.

Sometimes сложно can feel slightly more neutral or colloquial in modern speech, but the difference here is small.

They describe two different things.

• трудно describes your experience: it is hard for you
• сложный describes the film: the film is complicated

So the sentence is structured like this:

• Мне трудно... = I find it difficult...
• фильм слишком сложный = the film is too complicated

Using both is very natural, because one refers to the person’s difficulty and the other refers to the cause of that difficulty.