היא נוסעת לעבודה באופניים רק כשאין גשם.

Because it agrees with היא (she).

In the present tense, Hebrew verbs agree with gender and number:

• נוסע = masculine singular
• נוסעת = feminine singular
• נוסעים = masculine plural / mixed plural
• נוסעות = feminine plural

So with היא, you need נוסעת.

It comes from the verb לנסוע, which means to travel, to go by vehicle, or to ride/go somewhere.

In this sentence, נוסעת does not have to mean she is literally in motion right now. Hebrew present tense often covers:

• she travels
• she goes
• she commutes
• she is traveling

Here it is best understood as a habitual action: she does this as a regular practice.

Because the preposition ל־ (to) attaches directly to the noun.

So:

• ל־ = to
• עבודה = work
• לעבודה = to work

This is very common in Hebrew. Short prepositions such as ב־, כ־, and ל־ are usually attached to the following word.

In sentences like this, it usually just means to work in the normal idiomatic sense, like English go to work.

A useful thing to know: in unpointed Hebrew spelling, forms with ל־ do not always clearly show whether the noun is definite or not. So context does a lot of the work.

In everyday Hebrew, ללכת/לנסוע לעבודה is simply the normal way to say go/travel to work.

Here ב־ is being used to express the means of transportation: by bicycle / on a bicycle.

So:

• ב־ = by / in / with (depending on context)
• אופניים = bicycle / bicycle(s)
• באופניים = by bicycle

With לנסוע, Hebrew commonly uses ב for the vehicle or means:

• לנסוע באוטו = to go by car
• לנסוע באוטובוס = to go by bus
• לנסוע באופניים = to go by bike

Because אופניים is a plural-form noun in Hebrew, even when it refers to one bicycle.

This is similar to English words like scissors or glasses, which also look plural even when they refer to one object.

So learners often notice that אופניים ends like a plural word, and that is normal. Hebrew treats it grammatically as a plural-form noun.

Because it is made of two parts:

• רק = only
• כש־ = when

So:

• רק כש... = only when...

Then:

• אין גשם = there is no rain

Together, רק כשאין גשם means only when there is no rain.

Because אין is used to say that something does not exist or is not present.

So:

• אין גשם = there is no rain

By contrast, לא usually negates verbs or adjectives:

• היא לא נוסעת = she does not travel
• זה לא טוב = that is not good

So לא גשם would be wrong here.

A related point: Hebrew often does not use a present-tense verb for to be. So where English says there is no rain, Hebrew simply says אין גשם.

Yes. That is also very natural.

There is a slight difference in feel:

• כשאין גשם = when there is no rain
• כשלא יורד גשם = when it is not raining

The sentence you were given uses the first pattern, which is simple and very common. The second version focuses more directly on the action of rain falling.

Because גשם here is being used in a general sense: rain as a weather condition, not the rain as a specific known thing.

So:

• גשם = rain
• הגשם = the rain

In this sentence, the general meaning is what we want, so גשם without ה־ is natural.

The given order is very natural:

• היא נוסעת לעבודה באופניים רק כשאין גשם

But Hebrew can move parts around for emphasis. For example:

• רק כשאין גשם היא נוסעת לעבודה באופניים

That puts more emphasis on the condition.

What matters is that רק stays attached to the part it is limiting. Here רק belongs with כשאין גשם, so together they mean only when there is no rain.

Hebrew has more than one natural way to express this idea.

Two very common patterns are:

• לנסוע באופניים = to go / travel by bicycle
• לרכוב על אופניים = to ride a bicycle

So this sentence uses the first pattern:

• היא נוסעת ... באופניים

Another natural sentence would be:

• היא רוכבת לעבודה על אופניים

Both are good, but they use different verb-preposition combinations:

• נוסעת ב־
• רוכבת על־