אני בודקת את השרשרת כשאני מחליפה הילוך.

Because the speaker is feminine singular.

In the Hebrew present tense, the verb agrees with the subject in gender and number. So a female speaker says:

• אני בודקת = I check / I am checking
• אני מחליפה = I change / I am changing

A male speaker would say:

• אני בודק את השרשרת כשאני מחליף הילוך

Because the sentence has two clauses:

• אני בודקת את השרשרת = I check the chain
• כשאני מחליפה הילוך = when I change gear

Hebrew often repeats the subject in the second clause, especially in the present tense, because forms like בודקת and מחליפה show gender and number, but not person clearly enough on their own. Repeating אני makes it explicit that both actions are done by I.

No. Here את is not the pronoun you (feminine singular). In this sentence, את is the direct object marker.

So:

• את = you in some contexts
• את = marker of a definite direct object in other contexts

Here it is the second one, because it comes before השרשרת.

It marks the chain as the direct object of the verb בודקת.

Hebrew uses את before a definite direct object. English has no exact equivalent, so you usually do not translate it.

So:

• אני בודקת את השרשרת literally = I check [object marker] the chain
• natural English = I check the chain

Because השרשרת is definite (the chain), but הילוך here is indefinite (gear / a gear).

Hebrew normally uses את only before definite direct objects.

Compare:

• בודקת את השרשרת = checking the chain
• מחליפה הילוך = changing gear / a gear

If gear were definite, you could say:

• מחליפה את ההילוך = changing the gear

It is the Hebrew definite article, meaning the.

So:

• שרשרת = chain
• השרשרת = the chain

Hebrew attaches the directly to the noun as a prefix, instead of writing it as a separate word like English does.

כשאני means when I.

The כש־ part is a prefix meaning when (sometimes also as / while, depending on context), and it attaches directly to the next word.

So:

• כשאני = when I
• כשאתה = when you
• כשהוא = when he

This is completely normal Hebrew spelling.

Hebrew usually does not use a separate present-tense verb for am / is / are in sentences like this.

So:

• אני בודקת can mean I check or I am checking
• אני מחליפה can mean I change or I am changing

The present-tense verb form itself carries the meaning, and context tells you how to understand it.

It can mean either one.

Hebrew present tense usually does not make the same clear distinction that English makes between:

• I check
• I am checking

So the sentence could describe:

• a habitual action, or
• something happening right now

You understand which one is meant from the situation or wider context.

Hebrew commonly uses the singular in this expression:

• להחליף הילוך = to change gear / shift gear

Even though English often says shift gears, Hebrew naturally uses gear in the singular here to refer to switching from one gear to another.

So this is a normal idiomatic difference between the two languages.

Yes. The verb להחליף has a broad meaning: to change, switch, replace, exchange.

Its exact meaning depends on context. With הילוך, it naturally means:

• change gear
• shift gear(s)

So this is a normal use of the verb, even if the most basic dictionary meaning you first learned was something like replace.

Yes. That would also be natural Hebrew.

For example:

• אני בודקת את השרשרת כשאני מחליפה הילוך
• כשאני מחליפה הילוך, אני בודקת את השרשרת

Both are correct. The second version puts more focus on the when part.

In standard Hebrew, it is better to keep אני here.

The present-tense forms בודקת and מחליפה tell you feminine singular, but not clearly which person. Depending on context, they could refer to:

• I
• you (feminine singular)
• sometimes even she

So using אני makes the sentence clear and natural.