Ego saepe cogito utrum amica mea cras ventura sit.

Yes. The ending of cogito already tells you the subject is I, so ego is not grammatically necessary.

Latin often leaves subject pronouns out unless they are needed for:

• emphasis
• contrast
• clarity

So Ego saepe cogito... can feel like:

• I often think...
• As for me, I often think...

If you removed ego, the sentence would still be perfectly grammatical:

• Saepe cogito utrum amica mea cras ventura sit.

Utrum introduces an indirect yes/no question. Here it means whether.

So:

• utrum amica mea cras ventura sit = whether my girlfriend/female friend is going to come tomorrow

In Latin, indirect questions are commonly introduced by words like:

• utrum = whether
• num = whether
• interrogative words such as quis, quid, cur, ubi, etc.

You will often see utrum ... an ... when both alternatives are expressed:

• utrum veniat an maneat = whether she is coming or staying

In your sentence, only one side is stated, so utrum alone is enough.

Because utrum amica mea cras ventura sit is an indirect question, and indirect questions in Latin normally take the subjunctive.

So:

• direct question: Amica mea cras ventura estne? = Is my friend coming tomorrow?
• indirect question: cogito utrum amica mea cras ventura sit = I think/wonder whether my friend is coming tomorrow

The subjunctive here does not mean doubt or unreality by itself. It is there mainly because the clause is grammatically an indirect question.

That is a very common question.

Latin uses different constructions for different kinds of subordinate clauses:

• Indirect statement: usually accusative + infinitive

  • cogito amicam meam cras venturam esse = I think that my friend will come tomorrow

• Indirect question: interrogative word + subjunctive

  • cogito utrum amica mea cras ventura sit = I think/wonder whether my friend will come tomorrow

So the sentence is not saying I think that she will come, but I think about / wonder whether she will come. Because it is a question embedded inside another clause, Latin uses the indirect-question construction, not the infinitive construction.

Ventura is the future active participle of venio. It means about to come or going to come.

It is combined with a form of sum:

• ventura est = she is going to come / she will come

In your sentence, because this is an indirect question, est becomes subjunctive:

• ventura sit

So:

• amica mea cras ventura sit literally means something like
  my friend tomorrow may-be-about-to-come
• more natural English: my friend will come tomorrow or is going to come tomorrow

Ventura is:

• future active participle
• feminine
• singular
• nominative

It is feminine because it agrees with amica, which is feminine.

It is singular because amica is singular.

It is nominative because it describes the subject of sit.

So the agreement is:

• amica = feminine singular nominative
• mea = feminine singular nominative
• ventura = feminine singular nominative

All three go together:

• amica mea ventura = my female friend, about to come / going to come

They are nominative singular.

Here amica mea is the subject of sit:

• amica = friend
• mea = my

So:

• amica mea cras ventura sit = my friend may be going to come tomorrow

A native English speaker may expect something more like my friend to be clearly marked only by position, but Latin marks its role mainly by case ending, not by word order.

Saepe means often, and here it modifies cogito:

• Ego saepe cogito = I often think

Its position is natural, but Latin word order is flexible. You could also find:

• Saepe ego cogito...
• Ego cogito saepe...
• Cogito saepe...

Some word orders may sound more natural or more emphatic than others, but the meaning stays basically the same.

In general, adverbs like saepe often appear near the verb they modify, but they do not have to.

It can, depending on context.

The basic meaning of cogito is I think, I reflect, or I consider. With an indirect question, it can sound like:

• I think about whether...
• I consider whether...
• sometimes even I wonder whether...

That said, if someone wanted a very direct equivalent of English I wonder whether, Latin might also use other verbs depending on style and context, such as:

• quaero = I ask, investigate
• dubito = I doubt / am uncertain
• miror = I wonder, am surprised

So your sentence is understandable and grammatical, but cogito has a slightly more reflective feel than plain English I wonder.

Because Latin word order is much freer than English word order.

English depends heavily on position:

• subject + verb + object

Latin depends much more on:

• case endings
• verb endings
• agreement

So Latin can arrange words for emphasis, rhythm, or style.

In your sentence:

• Ego saepe cogito sets up the main idea first
• utrum signals that an indirect question is coming
• amica mea gives the subject of the subordinate clause
• cras adds the time
• ventura sit leaves the future idea and the verb to the end, which is very common in Latin

Putting the verb at the end of a clause is especially common in Latin, though not mandatory.

Usually no. Latin often follows sequence of tenses in subjunctive subordinate clauses.

With a present main verb:

• cogito utrum amica mea cras ventura sit
• I think/wonder whether my friend will come tomorrow

With a past main verb:

• cogitabam utrum amica mea cras ventura esset
• I was thinking/wondering whether my friend would come tomorrow

So:

• present main verb → often present subjunctive in the indirect question
• past main verb → often imperfect subjunctive

That is why sit appears here: it matches the present-tense main verb cogito.