Learn Hardcore Russian: Я не уверен, смогу ли я прийти на семинар завтра утром. - I’m not sure whether I will be able to come to the seminar tomorrow morning.

Уверен is the short-form adjective (краткая форма) used very often in predicative positions (after “to be” in English): Я (есть) уверен = “I am sure.”
Уверенный is the long-form adjective (полная форма) and typically modifies a noun: уверенный человек (“a confident person”).
You can say Я не уверенный in some contexts, but it sounds like you’re describing your general character (“I’m not a confident person”), not your certainty about a specific situation.

How would this change if the speaker is female or plural?

Short-form adjectives agree in gender/number:

male: Я не уверен
female: Я не уверена
plural: Мы не уверены

Why is there a comma after уверен?

Because this is a complex sentence: the main clause Я не уверен is followed by a subordinate clause that functions like an embedded question: смогу ли я прийти…
In Russian, such clauses are normally separated by a comma.

What does ли do here, and why is it placed after смогу?

Ли marks an indirect yes/no question: “whether/if.”
Russian usually puts ли right after the word being questioned/focused. Here, the “uncertain” part is whether I will be able → смогу ли (“whether I’ll be able”).
So: Я не уверен, смогу ли я… = “I’m not sure whether I’ll be able to…”

Could I also say Я не уверен, что смогу прийти…? Is it the same?

Both are correct, but the nuance differs:

Я не уверен, что смогу… = “I’m not sure that I’ll be able…” (treats it more like a statement you doubt)
Я не уверен, смогу ли я… = “I’m not sure whether I’ll be able…” (more explicitly an embedded yes/no question)

In everyday speech, both are common.

Why is it смогу and not могу?

Могу is present tense (“I can / I’m able [now]”).
Here you’re talking about tomorrow morning, so Russian uses the future of смочь (perfective): смогу = “I will be able.”

What’s the difference between смочь and мочь?

They’re the same “ability” idea but used differently in aspect/tense:

мочь (imperfective) → present: могу, past: мог
смочь (perfective) → future: смогу, past (successful ability as a completed result): смог

For future “will be able,” Russian normally uses смогу, not буду мочь.

Why is прийти used instead of приходить?

Прийти is perfective and refers to a single completed arrival (“to come/arrive [once]”).
Приходить is imperfective and can mean coming repeatedly or focusing on the process/habit.
For a one-time event (one seminar), прийти is the natural choice.

Why is я repeated: смогу ли я? Can it be omitted?

It can be omitted if it’s clear from context:

Я не уверен, смогу ли прийти… (also correct)

But repeating я is very common and sounds natural, especially in careful/neutral speech.

Why is it на семинар and not some other case?

With events like lectures/classes/seminars, Russian commonly uses на + Accusative to mean “to (attend/go to)”:

прийти на семинар (Accusative семинар after на)

If it were “into a room/building,” you might see в + Accusative instead (e.g., в аудиторию).

How does завтра утром work grammatically? What case is утром?

Завтра is an adverb (“tomorrow”).
Утром is historically the instrumental form of утро, but it’s used adverbially meaning “in the morning.”
Together завтра утром is simply “tomorrow morning.”

Is the word order flexible here?

Yes. Russian allows variation for emphasis:

Neutral: Я не уверен, смогу ли я прийти на семинар завтра утром.
Emphasize time: …смогу ли я прийти завтра утром на семинар.
Emphasize the event: …смогу ли я прийти на семинар. (time omitted)

The meaning stays basically the same; the focus shifts slightly.