Learn Hardcore Ukrainian: Я телефоную їй, поки стою в черзі. - I am calling her while I stand in the queue.

Because їй is dative case (“to her”) and the verb телефонувати (“to phone, call”) takes the dative: you phone to someone. Її is genitive/accusative (“her”/“hers”), used for possession or as a direct object.

I see her: Я бачу її.
I’m calling her: Я телефоную їй.
Her phone: Її телефон.

What case is в черзі and why does черга become черзі?

It’s the locative case, used after в/у to indicate location. The noun черга (queue/turn) in the locative singular becomes черзі. This is a regular consonant alternation: г → з before the front vowel -і (compare нога → на нозі, рука → у руці). So “in the queue” is в черзі.

Is the comma before поки necessary?

Yes. Поки introduces a subordinate time clause, so Ukrainian punctuation requires a comma:

Я телефоную їй, поки стою в черзі. If you put the time clause first, you still use a comma:
Поки стою в черзі, я телефоную їй.

Can I drop the subject pronoun я here?

Yes. Ukrainian often drops subject pronouns when the verb ending makes the subject clear. Both are fine:

Я телефоную їй, поки стою в черзі.
Телефоную їй, поки стою в черзі. Inside the поки-clause, я is also usually omitted if it’s the same subject.

Can I say Я їй телефоную instead of Я телефоную їй? Any difference?

Both are correct. Word order is flexible and used for emphasis.

Neutral/new info at the end: Я телефоную їй.
Emphasis on the person you’re calling: Я їй телефоную. You can even front it strongly for contrast: Їй я телефоную, а не йому.

Why в черзі and not у черзі?

Both в and у mean “in.” Ukrainian alternates them for euphony. After a vowel (as in стою), в often flows better: стою в черзі. But стою у черзі is also fine. Choose what sounds smoother and avoids awkward clusters or hiatus.

Does поки mean while or until?

On its own, поки means “while/as long as”: поки стою = “while I’m standing.”
With не, поки не means “until” (the не isn’t negation here):
Почекай, поки не закінчу = “Wait until I finish.” A near-synonym is доки; idiomatically, поки що means “for now/so far.”

Is телефонувати better than дзвонити?

Both are widely used for phone calls:

Neutral/standard: телефонувати комусь (dative).
Very common too: дзвонити комусь. You’ll also hear дзвонити до когось, though many style guides prefer the dative without до for calls. Only note that дзвонити у двері/в дзвони is “to ring a doorbell/bells.”

What about aspect? Could I use подзвоню here?

With поки you’re describing two simultaneous ongoing actions, so use the imperfective present: телефоную … поки стою.
The perfective future подзвоню (“I’ll give a call (once)”) emphasizes a single completed action and is not a natural fit with поки here. You would say, e.g., Я подзвоню їй, коли звільнюся (“I’ll call her when I’m free”).

Why no English-style “am -ing”? Is телефоную really “I am calling”?

Ukrainian has no separate progressive form. The simple present often covers English “am -ing” when context shows the action is in progress. So телефоную and стою naturally mean “I am calling” and “I am standing” here.

Could I use коли instead of поки?

You could, but it changes nuance:

Поки = “while, during the time that,” highlighting simultaneity.
Коли = “when/whenever,” more general and often read as habitual in the present:
Коли стою в черзі, телефоную їй ≈ “Whenever I stand in a line, I call her.”
For “right now, during this time,” поки is the best fit.

Does черга mean both queue and turn?

Yes.

Queue/line: стояти в черзі = “to stand in line.”
One’s turn: моя черга = “my turn,” ти на черзі = “you’re next (it’s your turn).” Note the prepositional difference: you stand “in” a queue (в/у черзі), but you are “on” the list as next (на черзі).

Can I say “в лінії” for “in line”?

No. Лінія is a drawn/abstract line, a power line, a production line, etc. For a queue, use черга: стояти в черзі.

Any quick pronunciation tips for tricky parts like їй and в черзі?

Їй sounds like “yee” with a short off-glide at the end; it’s one syllable.
В черзі flows as if the в were a light “v/w” attached to the next word: roughly “v CHER-zee.”
The ч is like “ch” in “church,” and з in черзі is “z” (not “zh”).
You’ll commonly hear stress on пóки, стою́, and че́рзі.