Если ракетка слишком тяжёлая, играть в теннис мне трудно.

Because если means if, and it introduces a subordinate clause:

Если ракетка слишком тяжёлая = If the racket is too heavy

Then the main clause follows:

играть в теннис мне трудно = playing tennis is difficult for me

Russian normally uses a comma before or after an если clause, depending on word order. Here the if clause comes first, so it is followed by a comma.

In the present tense, Russian usually does not use a separate word for is / am / are.

So:

ракетка тяжёлая literally looks like racket heavy, but it means the racket is heavy.

That is completely normal in Russian.

Because ракетка is the subject of the clause ракетка слишком тяжёлая.

It is the thing that is too heavy, so it stays in the nominative:

• ракетка = the racket
• тяжёлая = heavy

The adjective agrees with the noun:

• feminine singular noun: ракетка
• feminine singular adjective: тяжёлая

Because ракетка is a feminine noun.

Russian adjectives must agree with the noun in gender, number, and case:

• masculine: тяжёлый
• feminine: тяжёлая
• neuter: тяжёлое
• plural: тяжёлые

Since ракетка is feminine, the correct form is тяжёлая.

Слишком means too in the sense of more than is good or acceptable.

So:

слишком тяжёлая = too heavy

It usually goes directly before the adjective or adverb it modifies, just like in English:

• слишком тяжёлая = too heavy
• слишком быстро = too fast

In Russian, many games and sports are used with играть в.

So:

• играть в теннис = to play tennis
• играть в футбол = to play football/soccer
• играть в шахматы = to play chess

The noun after в usually takes the accusative case. For теннис, the accusative looks the same as the nominative, so you just see теннис.

Because Russian often uses an infinitive as the subject or topic of a sentence, especially with words like:

• трудно = difficult
• легко = easy
• интересно = interesting
• полезно = useful

So:

играть в теннис мне трудно literally means something like
To play tennis is difficult for me.

In more natural English, we usually say It is difficult for me to play tennis.

Because Russian uses the dative case with words like трудно, легко, можно, нельзя, интересно, and so on, when talking about the person affected.

So:

• мне трудно = it is difficult for me
• ему легко = it is easy for him
• нам интересно = it is interesting to us / for us

The pronoun мне is the dative form of я.

This is a very common Russian pattern:

• Мне трудно читать. = It is difficult for me to read.
• Ей легко готовить. = It is easy for her to cook.

Here трудно is a predicative word, often called a category of state word in Russian grammar. In practical learning terms, you can think of it as meaning:

• it is difficult
• hard

It does not behave like a normal adjective describing a noun. It is used in impersonal-style sentences such as:

• Мне трудно. = I’m having a hard time. / It is difficult for me.
• Здесь холодно. = It is cold here.
• Ему стыдно. = He is ashamed. / It is shameful for him.

So in this sentence, трудно is the core idea of the main clause: it is difficult.

Yes, мне трудно играть в теннис is also correct, and many learners may find it more straightforward.

Both mean the same thing:

• играть в теннис мне трудно
• мне трудно играть в теннис

The difference is mainly one of emphasis and information flow.

In играть в теннис мне трудно, the speaker puts playing tennis first, making that activity the topic.

In мне трудно играть в теннис, the speaker starts with for me, it is difficult, which may feel a bit more neutral.

Russian word order is more flexible than English, so both are natural depending on context.

Yes, but the meaning or style changes slightly.

• тяжёлая is the full adjective form
• тяжела is the short-form adjective

In this sentence, слишком тяжёлая is the most natural and expected version for everyday speech.

A version with short form, such as Если ракетка слишком тяжела..., is grammatical, but it sounds a bit more formal or literary to many learners.

For everyday Russian, слишком тяжёлая is the safer choice.

Normally если means if.

So here the meaning is conditional:

If the racket is too heavy, playing tennis is difficult for me.

It does not normally mean when in the simple time sense. For when, Russian usually uses когда.

Compare:

• Если ракетка слишком тяжёлая, играть трудно. = If the racket is too heavy, it is difficult to play.
• Когда ракетка у меня в руке, я нервничаю. = When the racket is in my hand, I get nervous.

Yes, it is natural Russian.

A native speaker might also say:

• Если ракетка слишком тяжёлая, мне трудно играть в теннис.
• Если ракетка слишком тяжёлая, мне трудно в неё играть. (less likely here)
• Если ракетка слишком тяжёлая, тяжело играть в теннис.

But your sentence is perfectly normal. The main thing to notice is the Russian pattern:

Если X..., [infinitive] мне трудно.

That structure is common and useful.