No estoy seguro de si elegiré esa optativa u otra más fácil.

In Spain, optativa is a very common word in school and university contexts.

• Una optativa = an optional subject / elective (a class you can choose, not a compulsory one).
• You might also hear asignatura optativa, but it’s often shortened simply to optativa.

So esa optativa means “that elective (class)”, not “optional thing” in general; it’s specifically about school/university subjects.

Yes, de is normally needed here.

The structure is:

• estar seguro de + [thing]
  
  • Estoy seguro de tu decisión. – I’m sure of your decision.
  • Estoy seguro de que vendrá. – I’m sure (that) he will come.

When what follows is an indirect question with si (“whether/if”), Spanish typically uses de si:

• No estoy seguro de si elegiré… – I’m not sure whether I will choose…

Without de, No estoy seguro si… sounds incomplete or foreign-influenced in standard European Spanish. You might occasionally hear it, but de si is the normal, correct form.

No, that doesn’t work as is. Two issues:

1. After estar seguro de que with a negative (no estoy seguro), Spanish usually takes the subjunctive, not the future indicative:

   • Natural: No estoy seguro de que vaya a elegir esa optativa.
   • Unnatural: No estoy seguro de que elegiré esa optativa.

2. There’s also a nuance difference:

   • No estoy seguro de si elegiré esa optativa…
     
     • Literally: I’m not sure whether I will choose that elective…
     • It’s an indirect question (si = whether).
   • No estoy seguro de que vaya a elegir esa optativa.
     
     • More like: I’m not sure (that) I’m going to choose that elective.
     • It sounds more like doubting the truth of the statement “I’m going to choose that elective” rather than presenting the choice as an open question.

Your original sentence is more naturally expressed with de si + [future / voy a + infinitive].

The future elegiré fits because the decision is in the future and not yet made.

Alternatives:

• No estoy seguro de si voy a elegir esa optativa…

  • Very common and natural, especially in spoken Spanish.
  • Similar meaning: I’m not sure if I’m going to choose…

• No estoy seguro de si elijo esa optativa…

  • This sounds odd; the present doesn’t match well with the idea of a future decision still under consideration.
  • Present would be more for fixed schedules or habitual actions.

So the best options are:

• No estoy seguro de si elegiré esa optativa…
• No estoy seguro de si voy a elegir esa optativa…

Both are correct; voy a elegir is slightly more colloquial and very frequent.

Elegir is mostly regular in meaning, but it has a spelling/stem change in some forms:

• Present:

  • yo elijo (g → j before o to keep the soft sound)
  • tú eliges, él/ella elige, etc.

• Future and conditional:

  • elegiré, elegirás, elegirá… – here the stem is eleg- with g, because it’s before e, which already gives a soft sound; no need for j.

• Past (preterite):

  • él eligió, ellos eligieron – vowel change e → i in some forms.

In your sentence, elegiré is a regular future form: infinitive elegir + future ending -é.

Spanish has a spelling/pronunciation rule for o (“or”):

• o becomes u before a word that starts with the /o/ sound (written o- or ho-)
  to avoid the sequence o o, which can be hard to hear.

So:

• o uno u otro – “one or the other”
• 7 u 8 – “7 or 8”
• esa optativa u otra más fácil – “that elective or another easier one”

If the next word doesn’t start with an /o/ sound, you use normal o:

• esa optativa o una obligatoria – “that elective or a compulsory one”

Otra is agreeing in gender and number with optativa, which is:

• feminine
• singular

So:

• esa optativa – that elective (feminine singular)
• otra (optativa) más fácil – another (elective) that is easier

The noun optativa is understood after otra and is not repeated:

• Full form (less natural): …esa optativa u otra optativa más fácil.
• Normal: …esa optativa u otra más fácil.

Adjectives that end in -l in their basic form usually do not change for gender in Spanish:

• un examen fácil – an easy exam (masculine)
• una asignatura fácil – an easy subject (feminine)

Same for many other adjectives:

• difícil: un problema difícil / una situación difícil
• igual: un caso igual / una respuesta igual

So más fácil is the same for masculine and feminine.
Here it just agrees in number (singular), but not in gender:

• singular: más fácil
• plural: más fáciles

Yes, that’s completely correct and very common:

• No sé si elegiré esa optativa u otra más fácil.
  = I don’t know whether I’ll choose that elective or another easier one.

No estoy seguro de si… and No sé si… are often interchangeable, but with a tiny nuance:

• No sé si… focuses on not knowing the answer yet.
• No estoy seguro de si… sounds a bit more like I’m uncertain / I haven’t decided / I’m not convinced yet.

In everyday speech, people use No sé si… a lot.

In this context, you need estar seguro:

• estar seguro (de algo) = to be sure / certain about something
  
  • No estoy seguro de si elegiré… – I’m not sure whether I’ll choose…

Ser seguro doesn’t mean the same in standard usage:

• ser seguro (for things/places) = to be safe / not dangerous
  
  • Este barrio es seguro. – This neighborhood is safe.
• For people, ser seguro is rare; you’d more often hear ser una persona segura (de sí misma) = self-confident.

So to talk about certainty/opinion, always use estar seguro (de), not ser seguro.

The word order in your sentence is the most natural:

• No estoy seguro de si elegiré esa optativa u otra más fácil.

You can technically move elegiré to the end:

• No estoy seguro de si esa optativa u otra más fácil elegiré.

…but this sounds unnatural and overly literary in modern Spanish, especially in European Spanish.

For everyday use, keep the standard SVO-like order inside the clause introduced by si:

• si + [subject (implied “yo”)] + verb + objects/complements
  → si elegiré esa optativa u otra más fácil.