Я не уверен, смогу ли я прийти на семинар завтра утром.

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.

Short-form adjectives agree in gender/number:

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

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.

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…”

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.

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.”

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.

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.

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.

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).

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.”

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.