Я пока не уверен, смогу ли я прийти вечером.

Here пока means for now / at the moment / yet.

So Я пока не уверен is literally something like I am not sure for now, but in natural English it is usually I’m not sure yet or I’m not sure at the moment.

This is a very common use of пока with temporary situations:

• Я пока дома. = I’m at home for now.
• Мы пока не знаем. = We don’t know yet.

It does not mean while in this sentence.

Because after Я you need the short-form adjective: уверен.

Russian often uses short-form adjectives after verbs like to be (even when быть is omitted in the present tense):

• Я уверен. = I am sure.
• Она уверена. = She is sure.
• Они готовы. = They are ready.

The full form уверенный usually describes a noun:

• уверенный человек = a confident person
• уверенный голос = a confident voice

So:

• Я не уверен = correct
• Я не уверенный = unnatural in this meaning

Уверен agrees with the speaker’s gender and number.

For a male speaker:

• Я пока не уверен...

For a female speaker:

• Я пока не уверена...

For plural:

• Мы пока не уверены...

This is because short-form adjectives change like this:

• masculine: уверен
• feminine: уверена
• neuter: уверено
• plural: уверены

Because смогу is the future tense of смочь and means will be able to / will manage to.

The sentence is about a possible action later, in the evening, so Russian uses the future:

• смогу прийти = will be able to come

Compare:

• Я могу прийти сейчас. = I can come now.
• Я не знаю, смогу ли прийти вечером. = I don’t know whether I’ll be able to come in the evening.

Also, смочь is perfective, so its present-looking forms actually have future meaning:

• смогу = I will be able
• сможешь = you will be able

They are closely related:

• мочь = to be able to, more general/imperfective
• смочь = to succeed in being able to / to manage to, perfective

In many future contexts, Russian strongly prefers смочь:

• Не знаю, смогу ли я прийти. = natural
• Не знаю, могу ли я прийти вечером. = more likely understood as whether I am allowed to or whether it is possible, depending on context

So in this sentence, смогу sounds most natural because the speaker is talking about whether circumstances will allow them to come later.

Ли introduces an indirect yes/no question, similar to English whether or sometimes if.

So:

• сможу ли я прийти = whether I will be able to come

Compare:

• Я не знаю, придёт ли он. = I don’t know whether he will come.
• Спроси, дома ли она. = Ask whether she is at home.

In this sentence, the whole second part means:

• I’m not sure whether I’ll be able to come in the evening.

Because ли usually comes right after the word it is attached to, and that word is often the key element being questioned.

Here the main uncertainty is whether I will be able:

• смогу ли я прийти

This is the normal neutral order.

If you move ли, the emphasis changes, and some versions sound unnatural or highly marked. For example:

• Я пока не уверен, смогу ли я прийти вечером. = neutral, standard
• Я пока не уверен, я ли смогу прийти вечером. = sounds like whether it is I who will be able to come, which is a different emphasis

So смогу ли я is the normal pattern for whether I will be able.

Because Russian often keeps the subject in both clauses when it is part of each clause’s grammar.

The sentence has two clauses:

1. Я пока не уверен
2. смогу ли я прийти вечером

That second я belongs to the subordinate clause.

You can sometimes omit it if the meaning is clear:

• Я пока не уверен, смогу ли прийти вечером.

That is also natural and common.

Including я can make the clause a little clearer or slightly more explicit. Both versions work.

Because прийти is perfective, and here the speaker means to come on one specific occasion.

Compare:

• прийти = to come / arrive once
• приходить = to come regularly / to be in the process of coming / repeated coming

In this sentence, the meaning is:

• whether I’ll be able to come this evening
  not
• whether I’m generally able to come evenings

So прийти is the natural choice.

Examples:

• Я смогу прийти завтра. = I’ll be able to come tomorrow.
• Я часто прихожу рано. = I often come early.

Because Russian often uses the instrumental case to express time when, especially with parts of the day.

So:

• утром = in the morning
• днём = during the day / in the daytime
• вечером = in the evening
• ночью = at night

That is why the sentence says:

• прийти вечером = to come in the evening

This is a fixed, very common pattern.

Sometimes yes, depending on context, but not always exactly.

Вечером literally means in the evening.
In many real situations, if the evening is understood to be this evening, English may naturally use tonight:

• Я не уверен, смогу ли я прийти вечером. = I’m not sure whether I’ll be able to come tonight.

But grammatically, Russian is saying in the evening, not specifically the English word tonight.

The comma separates the main clause from the subordinate clause.

Main clause:

• Я пока не уверен = I’m not sure yet

Subordinate clause:

• смогу ли я прийти вечером = whether I’ll be able to come in the evening

Russian normally uses a comma before subordinate clauses like this, especially when they are introduced by words such as что, если, когда, or structures with ли.

Yes. Both are possible, but the nuance is slightly different.

• пока не уверен = not sure for now / at the moment
• ещё не уверен = not sure yet

In many contexts they are very close.

Compare:

• Я пока не уверен, смогу ли я прийти вечером.
• Я ещё не уверен, смогу ли я прийти вечером.

Both are natural.
Пока often highlights the current temporary state a bit more, while ещё more directly emphasizes still not yet.

It is neutral and very natural in everyday speech and writing.

It is not especially formal, but it is also not slangy or casual in a rough way. You could use it:

• with friends
• with colleagues
• in messages
• in normal conversation

If you wanted to sound a little more conversational, you might omit the second я:

• Я пока не уверен, смогу ли прийти вечером.

But the original sentence is completely normal.