Learn Hardcore Russian: Осторожная водительница медленно едет по узкому шоссе. - A careful female driver is driving slowly along the narrow highway.

The basic form of the adjective is осторожный (careful). For feminine, singular, nominative it changes to осторожная (ending -ая) to match водительница.

Why is водительница used instead of водитель?

Водитель is grammatically masculine and usually means driver (gender-neutral in many contexts, especially if gender is unknown or not important).

Водительница is the explicitly feminine form and clearly means female driver:

водитель
- feminine suffix -ниц(а) → водительница

So the sentence wants to make it clear that the driver is a woman.

What case is по узкому шоссе, and why is that case used?

По узкому шоссе is in the dative case.

The preposition по often governs the dative when it means along / over / on (while moving).
- идти по улице – to walk along the street
- ехать по мосту – to drive over the bridge

Here, по узкому шоссе means along the narrow highway, so по takes the dative:

узкий → dative masculine/neuter singular узкому
шоссе is dative as well, but it does not change its form.

Why is it узкому, not узком?

The adjective узкий (narrow) has different endings for dative vs. prepositional:

Dative masculine/neuter singular: -ому → узкому
Prepositional masculine/neuter singular: -ом → узком

With the meaning moving along / over something, по normally takes the dative, so the correct form is:

по узкому шоссе, not по узком шоссе.

What is the gender and declension of шоссе?

Шоссе is:

Neuter gender.
Indeclinable in the singular: its form is the same in all cases (nominative, genitive, dative, etc.) → always шоссе.

You know it is neuter mainly from:

Its pattern as a loanword ending in -е (like кафе, такси, метро, шоссе).
The adjective узкому, which is masculine/neuter dative, agreeing with a neuter noun here.

What tense and aspect is едет, and what verb is it from?

Едет is:

Present tense, 3rd person singular.
From the verb ехать (imperfective).
It describes an ongoing action right now: is driving / is going by vehicle.

So медленно едет ≈ is going/driving slowly (right now).

Why do we use едет (from ехать) and not ездит (from ездить)?

Russian has two basic verbs for going by vehicle:

ехать – unidirectional, usually one trip in one direction, often “right now”.
- Она едет в город. – She is going (driving) to the city (now).
ездить – multidirectional, used for repeated / habitual movement or in general statements.
- Она ездит в город каждый день. – She goes (drives) to the city every day.

In your sentence, we are describing what is happening at this moment:
Осторожная водительница медленно едет по узкому шоссе. → she is currently driving slowly along the narrow highway, so едет from ехать is correct.

Why is ехать used and not идти?

Russian distinguishes between going on foot and going by vehicle:

идти / ходить – to go on foot (walk).
ехать / ездить – to go by vehicle (car, bus, train, etc.).

Since we are talking about a driver on a highway, she is clearly in a vehicle, so ехать (and its form едет) is used, not идти.

Can the word order change, for example to Осторожная водительница едет медленно по узкому шоссе? Does the meaning change?

Yes, Russian word order is flexible, so you can say:

Осторожная водительница медленно едет по узкому шоссе.
Осторожная водительница едет медленно по узкому шоссе.
Осторожная водительница едет по узкому шоссе медленно.

All are grammatically correct and basically mean the same thing.
The default, neutral order is with the adverb медленно just before the verb (медленно едет), but moving медленно slightly changes emphasis:

медленно едет – slight focus on the manner of movement.
едет медленно – can sound a bit more contrastive or emphatic (not fast, but slowly).

For most learners’ purposes, they are interchangeable here.

How would the sentence change if the driver were male?

You would normally use the masculine form of the noun and adjective:

Осторожный водитель медленно едет по узкому шоссе.

Changes:

водительница → водитель (masculine).
осторожная → осторожный to agree with the masculine noun.

Everything else stays the same.

What are the stresses in this sentence?

Stresses (marked with ´ over the stressed vowel):

осторо́жная – o-sto-rózh-na-ya
води́тельница – vo-dí-tel’-ni-tsa
ме́дленно – méd-len-no
е́дет – yé-det
по – (monosyllabic, effectively stressed)
у́зкому – úz-ko-mu
шоссе́ – sha-ssé

Pronouncing the stresses correctly will make the sentence sound natural.

Is шоссе exactly the same as road? Can I use дорога instead?

Шоссе is closer to highway / main road – a major paved road for faster traffic.

Дорога is more general: road, way, route. You could say:

Осторожная водительница медленно едет по узкой дороге.

This is also correct, but it shifts the image from a highway to just a narrow road (could be any kind of road, not necessarily a highway).

Does осторожная mean the same as аккуратная, or is there a difference?

Both can describe a “careful” person, but there is a nuance:

осторожная – cautious, avoids danger or risk; thinks about safety.
аккуратная – tidy, neat, precise, does things carefully and neatly.

For a driver, осторожная водительница emphasizes that she is cautious and safe on the road, which fits the context better than аккуратная водительница (which would more suggest she is neat, orderly, precise).