Если в тексте есть непонятное выражение, я открываю словарь.

Если means if.

It introduces a condition:

• Если в тексте есть непонятное выражение = If there is an unfamiliar expression in the text
• я открываю словарь = I open the dictionary

So the whole sentence has the common Russian pattern:

Если + condition, main clause

This is directly comparable to English If ..., ...

In Russian, a subordinate clause introduced by если is normally separated from the main clause by a comma.

So:

• Если в тексте есть непонятное выражение, я открываю словарь.

The first part is the if-clause, and the second part is the main statement.

This comma is required in standard writing.

Because в can take different cases depending on meaning.

Here в тексте means in the text in the sense of location, so Russian uses the prepositional case:

• текст → в тексте

Compare:

• в тексте = in the text where something is located
• в текст would suggest motion into the text, which is not the meaning here

So в тексте is correct because the expression is located inside the text.

Here есть means there is / there exists.

So:

• в тексте есть непонятное выражение = there is an unfamiliar expression in the text

Russian often omits есть when saying something simply is something, but in expressions of existence like there is / there are, есть is very common.

Compare:

• Это словарь. = This is a dictionary.
• На столе есть словарь. = There is a dictionary on the table.

In your sentence, есть emphasizes existence: an unfamiliar expression appears in the text.

Because the adjective must agree with the noun in gender, number, and case.

The noun выражение is:

• neuter
• singular
• here in the nominative case

So the adjective must also be neuter singular nominative:

• непонятное выражение

Compare:

• непонятный текст — masculine
• непонятная фраза — feminine
• непонятное выражение — neuter

That -ое ending shows neuter agreement.

Непонятное literally means not understandable or unclear.

In natural English, depending on context, it could mean:

• unfamiliar
• unclear
• hard to understand

So непонятное выражение can be understood as:

• an expression I don’t understand
• an unclear expression
• an unfamiliar expression

The prefix не- simply negates понятное:

• понятный = understandable, clear
• непонятный = not understandable, unclear

Выражение usually means expression or phrase, while слово means word.

So the sentence specifically says:

• not just a single unknown word
• but possibly a whole expression, phrase, or set phrase

Examples:

• непонятное слово = an unfamiliar word
• непонятное выражение = an unfamiliar expression

Russian distinguishes these just like English does.

Открываю is imperfective present tense, and here it expresses a habitual action:

• Whenever this happens, I open the dictionary
• If there is an unfamiliar expression, I open the dictionary

This is not talking about one single future event. It describes what the speaker generally does.

Compare:

• я открываю словарь = I open / I usually open / I am in the habit of opening
• я открою словарь = I will open

So открываю is the natural choice for a general rule or repeated behavior.

Словарь is the direct object of открываю (I open what?), so it is in the accusative case.

However, for many inanimate masculine singular nouns, the accusative looks exactly like the nominative.

So:

• nominative: словарь
• accusative: словарь

That is why the form does not change.

Compare with another inanimate masculine noun:

• я читаю текст — текст is accusative, but looks the same as nominative

Yes, it often could be omitted in Russian if the meaning is clear from the verb:

• Если в тексте есть непонятное выражение, открываю словарь.

That can sound natural in the right context.

However, я is included here because:

• it makes the sentence clearer for learners
• it can add a little emphasis on I
• Russian often includes subject pronouns when the speaker wants to be explicit

So both are possible, but the version with я is completely normal.

In sentences like this, если can often be understood as if or whenever, depending on context.

Because the main verb is открываю and describes a repeated habit, the whole sentence really has the sense of:

• If / whenever there is an unfamiliar expression in the text, I open the dictionary.

So even though если literally means if, the sentence as a whole can express a general repeated situation.

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

The original sentence:

• Если в тексте есть непонятное выражение, я открываю словарь.

This is a very natural, neutral way to say it.

You could also say:

• Я открываю словарь, если в тексте есть непонятное выражение.

This is also correct, but now the main action comes first, and the condition comes after.

So the meaning stays basically the same, but the emphasis can shift slightly depending on which part comes first.