Iba a estudiar español por la noche; sin embargo, me sentí muy cansado.

Iba a estudiar literally means “I was going to study” and is used for an intention or plan in the past, especially one that did not end up happening (or at least was interrupted).

• Iba a estudiar español por la noche = I was going to study Spanish at night (but then something changed).
• Estudiaría español (conditional) = I would study Spanish (often hypothetical; depends on some condition).
• Estudiaré español (future) = I will study Spanish (future plan, not past).

So iba a + infinitive is the natural way to talk about a plan or intention you had at some point in the past.

In Spanish, the construction ir a + infinitive is a fixed pattern that can mean:

1. Near future (in the present or past):

   • Voy a estudiar = I’m going to study.
   • Iba a estudiar = I was going to study.

2. Movement toward an action:

   • Fui a estudiar = I went to study.

In both uses, the preposition a is required.
You can’t drop it: ✗ iba estudiar is incorrect. It must be ✓ iba a estudiar.

Iba (imperfect) focuses on an ongoing state, plan, or intention in the past:

• Iba a estudiar español por la noche = At that time, I was going to / was planning to study Spanish that night.

Fui (preterite) would describe a completed action:

• Fui a estudiar español por la noche = I went to study Spanish at night (and that action itself is completed).

Here, the speaker wants to highlight the intention that got interrupted (feeling tired) rather than a completed “going” event, so iba is the natural choice.

Spanish usually omits subject pronouns because the verb ending already shows the subject.

• Iba a estudiar is clearly first person singular (yo), so yo isn’t needed.
• Yo iba a estudiar is also correct, but it adds emphasis, like “I was going to study”, perhaps contrasting with someone else.

In normal, neutral speech, Iba a estudiar without yo is most natural.

Both are understood and used in Latin America, but there are preferences:

• Por la noche

  • Very standard and widely used in both Spain and Latin America.
  • Means “at night / during the night” in a general way.
  • Sounds neutral in most regions.

• En la noche

  • Also common in much of Latin America, especially in everyday speech.
  • Often feels a bit more specific, like “in the evening/night (at that time).”

In your sentence, por la noche is perfectly natural and probably the safest “textbook” choice:
Iba a estudiar español por la noche.

Both express contrast, but they’re a bit different in tone:

• Pero ≈ but

  • More informal, very common in speech.
  • Strong, direct contrast: Quería estudiar, pero estaba cansado.

• Sin embargo ≈ however / nevertheless

  • A bit more formal or “connected-speech” style.
  • Often used to connect sentences or clauses in writing.
  • Fits very well with a semicolon or a new sentence:
    
    • Iba a estudiar español por la noche; sin embargo, me sentí muy cansado.

Using sin embargo here sounds slightly more formal or written than pero, but both are correct:

• …por la noche, pero me sentí muy cansado.
• …por la noche; sin embargo, me sentí muy cansado.

Yes, you have options in Spanish punctuation, similar to English:

• Semicolon (;)

  • Connects two closely related sentences.
  • Works very well with sin embargo:
    
    • …por la noche; sin embargo, me sentí muy cansado.

• Period (.)

  • Also correct, slightly more separate:
    
    • …por la noche. Sin embargo, me sentí muy cansado.

• Comma (,)

  • In practice, many native speakers write:
    
    • …por la noche, sin embargo, me sentí muy cansado.
  • Some style guides prefer a stronger break (semicolon or period), but the comma is common in real life.

So the semicolon isn’t mandatory, but it’s a good, clear choice.

Sentí and me sentí are different verbs:

• Sentir (no pronoun) = to feel something (an object):

  • Sentí frío. = I felt cold.
  • Sentí un dolor. = I felt a pain.

• Sentirse (reflexive: me sentí) = to feel a certain way / in a certain state:

  • Me sentí muy cansado. = I felt very tired.
  • Me siento bien. = I feel good.

In me sentí muy cansado, you’re talking about your inner state (how you felt), so the reflexive form sentirse with me is correct and natural.

Both are grammatically correct, but they focus on different aspects:

• Me sentí muy cansado. (preterite)

  • A specific moment when you felt tired.
  • A change or event: “At that point, I (suddenly) felt very tired.”

• Me sentía muy cansado. (imperfect)

  • A continuous or repeated feeling over some period:
    
    • “I was feeling very tired (for a while).”

In your sentence, the tiredness appears as a specific reaction that affected your plan, so me sentí (preterite) is the natural choice.

In Spanish:

• Muy is used with adjectives and adverbs:

  • muy cansado = very tired
  • muy rápido = very fast
  • muy grande = very big

• Mucho is used with verbs or nouns:

  • Te extraño mucho. = I miss you a lot.
  • Tengo mucho trabajo. = I have a lot of work.

Since cansado is an adjective (“tired”), you must use muy, not mucho:
me sentí muy cansado.

Yes, cansado agrees with the subject’s gender and number:

• A man:
  
  • Me sentí muy cansado.
• A woman:
  
  • Me sentí muy cansada.
• A group of men / mixed group:
  
  • Nos sentimos muy cansados.
• A group of women:
  
  • Nos sentimos muy cansadas.

So if the speaker is female, the sentence should be:
Iba a estudiar español por la noche; sin embargo, me sentí muy cansada.

You can say both, but they’re not identical:

• Me sentí muy cansado.

  • Focuses on the moment you experienced the feeling.
  • More like “I (suddenly) felt very tired.”

• Estaba muy cansado.

  • Describes a state or condition in the past.
  • More like “I was very tired (at that time/in general).”

In the original sentence, me sentí highlights the feeling appearing and affecting your plan.
If you say estaba muy cansado, it sounds more like you’re just describing how you were that night.

No. In Spanish, names of languages are written with lowercase:

• español, inglés, francés, alemán, etc.

So the correct form is:

• Iba a estudiar español por la noche; sin embargo, me sentí muy cansado.
  (No capital E in español.)