Learn Hardcore Russian: Мой друг осторожный водитель.

So you don’t say есть (is/are) here.
If you add a verb like является (is, constitutes), then you must change the case (see another question below).

Why isn’t there a word for “a” before водитель?

Russian has no articles (a, an, the).

Whether English would use “a” or “the” is understood from context in Russian, not from a specific word. So:

Мой друг осторожный водитель.
can mean:
- My friend is a careful driver.
  (most natural translation here)

There is simply no Russian equivalent of “a”.

What cases are used in this sentence?

All the main words are in the nominative case:

Мой – nominative masculine singular possessive pronoun
друг – nominative masculine singular noun (subject)
осторожный – nominative masculine singular adjective (agrees with водитель)
водитель – nominative masculine singular noun (predicate)

Pattern: Subject (нominative) + Predicate noun/adjective (nominative) in the present tense.

Why do мой, осторожный, and водитель have these endings?

Because they agree in gender, number, and case with the nouns they refer to.

друг – masculine, singular, nominative
водитель – masculine, singular, nominative

So we use masculine, singular, nominative forms:

мой (my – masculine, singular, nominative)
- моя (feminine)
- моё (neuter)
- мои (plural)
осторожный (careful – masculine, singular, nominative)
- осторожная (feminine)
- осторожное (neuter)
- осторожные (plural)
водитель is already in its nominative form.

Everything matches the masculine subject друг.

Why is it осторожный, not осторожно?

осторожный is an adjective: careful
осторожно is an adverb: carefully

Here, осторожный describes the noun водитель (driver), so you need an adjective:

осторожный водитель – a careful driver (adjective + noun)

If you want to say “drives carefully”, you use the adverb:

Мой друг водит осторожно. – My friend drives carefully.

Why is друг used? What if the friend is female?

друг is “friend” in the masculine. It usually refers to a male friend.

If your friend is female, you normally say:

Моя подруга осторожный водитель. – My (female) friend is a careful driver.

Note the changes:

моя instead of мой (feminine)
подруга instead of друг (female friend)

The predicate осторожный водитель can stay masculine grammatically, because водитель is a masculine noun by form (see next question), though in speech many people would reword to avoid the mismatch, for example:

Моя подруга очень осторожно водит. – My (female) friend drives very carefully.

Is водитель masculine or feminine? Can it refer to a woman?

Grammatically, водитель is masculine:

ends in a soft sign -ь
adjectives and pronouns agree with it as masculine:
- осторожный водитель, хороший водитель, этот водитель

However, it can refer to a person of any sex. Context tells you whether the driver is male or female.

If you need to emphasize a female driver, you might say:

женщина-водитель – woman driver

But grammatically, водитель itself stays masculine.

Can I put a dash: Мой друг — осторожный водитель?

Yes. Both versions are correct:

Мой друг осторожный водитель.
Мой друг — осторожный водитель.

The dash is often used in Russian between two nouns (or a noun + adjective phrase) in the nominative to highlight the identity or classification: X is Y.

In simple spoken language, the dash is often omitted; in written or more careful style, many people prefer to include it.

Why isn’t it Мой друг осторожный водитель + ом (instrumental)?

You might know that professions often appear in the instrumental case with forms of быть:

Он был водителем. – He was a driver.
Он будет водителем. – He will be a driver.
Он является водителем. – He is a driver. (formal)

In the present tense without an explicit verb, Russian normally uses the nominative for the predicate:

Он водитель. – He is a driver.
Мой друг осторожный водитель.

So:

no visible verb → nominative: водитель
verb like был / будет / является → instrumental: водителем

How would this sentence look in the past or future tense?

You then add a verb and usually switch the predicate to instrumental:

Past:
- Мой друг был осторожным водителем.
  My friend was a careful driver.
Future:
- Мой друг будет осторожным водителем.
  My friend will be a careful driver.

Changes:

add был / будет
осторожный водитель → осторожным водителем (instrumental)

Can I express the same idea using “drives carefully” instead of “careful driver”?

Yes, a very natural way is:

Мой друг водит очень осторожно.
My friend drives very carefully.

Compare:

Мой друг осторожный водитель. – focuses on what kind of driver he is.
Мой друг водит очень осторожно. – focuses on how he drives.

Both are correct; they simply emphasize different things.

Can I change the word order, like Осторожный водитель — мой друг?

Yes, Russian word order is quite flexible. For example:

Осторожный водитель — мой друг.
Мой друг — осторожный водитель.

Both are grammatical. The first position tends to get more emphasis:

Осторожный водитель — мой друг.
Emphasis on “the careful driver” (who is that? → it’s my friend).

In your original sentence Мой друг осторожный водитель, the focus is more on “my friend” and what kind of driver he is.

All of these are acceptable; the differences are mostly about nuance and emphasis.