Learn Hardcore Russian: Сейчас я не занят и свободен, я могу говорить с тобой по телефону. - Right now I am not busy and I am free; I can talk with you on the phone.

Сейчас focuses on the current moment in time, like “right now, at this moment”.
Сейчас я не занят… = Right now I’m not busy…
Теперь is more about a change compared to before: “now (as opposed to earlier)”.
Теперь я не занят… suggests “I used to be busy, but now I’m not.”

In your sentence, Сейчас is the most neutral and common choice, but Теперь would work if you want to stress a contrast with the past (e.g. “I’ve just finished, now I’m free.”).

2. Why say both не занят and свободен? Aren’t they basically the same?

They’re close, but not identical:

не занят = not busy.
It just denies being occupied with something.
свободен = free / available.
This is positive: you’re free for something (here: to talk).

Saying я не занят и свободен strengthens the idea:

Not only am I not busy,
but I am positively free and available.

In everyday speech, most natives would often use just one:

Я сейчас свободен, могу поговорить по телефону.
Я сейчас не занят, могу поговорить по телефону.

Using both is fine and a bit emphatic.

3. Why я не занят, not я не занятый?

In Russian, short-form adjectives are often used to describe temporary states:

я занят / не занят – I’m (not) busy (right now)
я свободен – I’m free (right now)

Занятый is the full-form adjective and behaves more like an attribute:

занятый человек – a busy person
Я очень занятый человек. – I’m a very busy person (in general).

So:

Я не занят. = correct, natural for “I am not busy (at the moment).”
Я не занятый. sounds strange; you almost never negate that full adjective like this in predicative position.

4. How do занят / занята / занято / заняты change with gender and number?

These are short-form adjectives, and they agree with the subject:

masculine singular:
Я занят. – I (male) am busy.
Он занят. – He is busy.
feminine singular:
Я занята. – I (female) am busy.
Она занята. – She is busy.
neuter singular:
Окно занято. – The window (slot/time) is taken.
plural (any gender):
Мы заняты. – We are busy.
Они заняты. – They are busy.

In your sentence, я не занят implies the speaker is male.
A female speaker would say: Сейчас я не занята и свободна…

5. Is the comma before я могу говорить correct? Could it be two separate sentences?

Yes, the comma is grammatically correct in Russian:

Сейчас я не занят и свободен, я могу говорить с тобой по телефону.

This is a comma splice by English standards, but in Russian it’s common: two related independent clauses joined by a comma.

You have three main options:

Comma (as in the original):
Natural in informal writing/speech.
Add a conjunction:
Сейчас я не занят и свободен, и я могу говорить с тобой по телефону.
This is correct but sounds a bit heavy and repetitive.
Split into two sentences (very natural):
Сейчас я не занят и свободен. Я могу говорить с тобой по телефону.

For clear, careful writing, option 3 is usually best.

6. Can I drop the second я and say …, могу говорить с тобой по телефону?

Yes, and that will actually sound more natural:

Сейчас я не занят и свободен, могу говорить с тобой по телефону.

Russian often omits repeated subjects when it’s clear who is acting. Both versions are correct:

With repetition: …, я могу говорить… – slightly more explicit.
Without: …, могу говорить… – smoother, very typical in speech.

In neutral conversation, most natives would prefer the version without the second я.

7. Why я могу говорить, not just я говорю?

Я говорю с тобой по телефону means “I am (now) talking with you on the phone.”
It describes an action that is actually happening.

Я могу говорить с тобой по телефону means “I can / I’m able to talk with you on the phone (now).”
It emphasizes possibility/availability, not the action itself.

In English terms:

I talk with you on the phone vs.
I can talk with you on the phone (if you want).

Given the first part “I’m not busy and free now”, it makes sense to express ability/availability, so могу говорить is the right choice.

8. Could I use поговорить or разговаривать instead of говорить here?

You could, but the nuance changes slightly:

говорить с тобой по телефону
– neutral “to talk/speak with you on the phone.”
поговорить с тобой по телефону (perfective)
– “to have a (some) talk with you on the phone.”
– focuses on having a conversation as a complete event.
Example:
Я могу поговорить с тобой по телефону. – I can (take the time to) talk with you on the phone.
разговаривать с тобой по телефону
– emphasizes the process of chatting / conversing.
Example:
Я могу разговаривать с тобой по телефону. – I can be on the phone talking with you (I’m allowed/able to talk for some time).

In your sentence, all three are grammatically fine:

Most neutral: могу говорить с тобой по телефону.
Slight sense of “to have a talk”: могу поговорить с тобой по телефону.
More process-focused: могу разговаривать с тобой по телефону.

9. Why is it с тобой, not тебя or тебе?

The verb говорить (and разговаривать) normally takes preposition + Instrumental when you say “talk with someone”:

говорить с кем? – talk with whom?
with Instrumental case after с.

Pronoun forms:

ты → тобой (Instrumental)
я → мной
он → ним / им, etc.

So:

говорить с тобой – to talk with you (using с + тобой, Instrumental).

Тебя (Genitive/Accusative) and тебе (Dative) would be wrong here because the preposition с requires the Instrumental in this meaning.

10. Why exactly is it тобой and not твоей?

Тобой is the Instrumental form of the pronoun ты (you – singular, informal).

ты → тобой (Instrumental)
твоей is a form of the possessive adjective твой (your), feminine Genitive/Dative/Instrumental.

Compare:

с тобой – with you (person)
с твоей подругой – with your (female) friend

So:

говорить с тобой – correct: “talk with you”
говорить с твоей – incomplete; you’d have to say with your what?

11. Why do we say по телефону and not на телефоне?

In Russian, по телефону is the standard way to say “on the phone / by phone”:

говорить по телефону – talk on the phone
разговаривать по телефону – have a conversation on the phone

The preposition по here indicates means or channel of communication.

На телефоне is not used for “on the phone” in the English sense. It might appear in slang for something like “on someone else’s phone (device)” or literally “on top of the phone”, but not as a normal equivalent of by phone.

So for ordinary “talk on the phone”, always use:

по телефону
or with other devices: по скайпу, по видеосвязи, etc.

12. Could I change the word order, for example Я сейчас не занят… or Сейчас я свободен и не занят?

Yes, Russian word order is fairly flexible. All of these are correct:

Сейчас я не занят и свободен…
– neutral; slight emphasis on the time (now).
Я сейчас не занят и свободен…
– still neutral; slight emphasis on “I” (the subject).
Сейчас я свободен и не занят…
– just reverses the adjectives; a bit stylistic.

All three would be understood the same way. In everyday speech, Я сейчас не занят, могу поговорить по телефону is probably the most common-sounding pattern.

13. Would a native actually say this sentence as-is, or is there a more natural version?

Your sentence is correct and understandable, but a native speaker would usually make it slightly shorter and avoid repetition:

Very natural versions:

Сейчас я свободен, могу говорить с тобой по телефону.
Я сейчас не занят, могу поговорить с тобой по телефону.
Я сейчас свободен, можем поговорить по телефону. (we can talk…)

Keeping both не занят и свободен is possible but feels a bit wordy in casual speech unless you want to be especially emphatic.

14. Where is the stress in the words сейчас, занят, свободен, говорить, телефону?

Pronunciation with stress (stressed syllable in bold caps):

сейЧАС – сейчас
ЗАнят – занят (male form; female: заНЯТА – заНЯта)
своБОден – свободен (female: своБОднА – свободна)
говорИть – говорить (infinitive; stress stays on -И- when conjugated: я говорЮ, ты говорИшь, etc.)
телеФОну – телефону

Pay special attention to:

сейчас – many learners wrongly stress the first syllable (СЕйчас). Correct is сейЧАС.
занят – stress on the first syllable in masculine, but it moves in some other forms (заНЯта, занЯты).