Eu vou pagar pelo curso de português amanhã.

In Brazilian Portuguese, the most common way to talk about a planned future action is ir (present) + infinitive: eu vou pagar = I’m going to pay.
Pagarei is correct, but it sounds more formal, written, or emphatic (and is less common in everyday speech in Brazil).

You can often omit it: Vou pagar pelo curso de português amanhã.
Portuguese verbs usually indicate the subject clearly, so eu is optional. People keep eu for emphasis/contrast (e.g., Eu vou pagar, not someone else).

Pelo = por + o (a contraction).

• por can mean for (among other meanings)
• o = the (masculine singular)
  So pagar pelo curso literally is to pay for the course.

Grammatically it’s understandable, but in normal Portuguese you virtually always contract it:

• Natural: pagar pelo curso
• Unnatural/rare: pagar por o curso

Because pagar por is the standard way to say pay for (something) in Portuguese:

• pagar pelo curso = pay for the course
  Pagar para usually means pay to (someone) or pay in order to (depending on context), and it doesn’t fit the “pay for a thing/service” meaning as well.

Portuguese typically uses:

• pagar por
  • thing/service: Vou pagar pelo curso.
• pagar (algo) a/para alguém
  • recipient (varies by speaker/region):
    
    • Vou pagar o curso para a escola. (pay the school for it)
      Many Brazilians also just say pagar X pra alguém in casual speech.

de commonly links nouns, similar to of in English.
So curso de português = Portuguese course / course of Portuguese. This is the normal way to express the subject/topic of a course.

Both are possible, with a small nuance:

• pagar pelo curso = paying for the course (a specific one, implied by context)
• pagar por um curso = paying for a course (less specific)
  In your sentence, pelo already includes o (the), so it’s already definite.

Time words are flexible, but these are all natural:

• Vou pagar pelo curso de português amanhã. (common, neutral)
• Amanhã vou pagar pelo curso de português. (tomorrow emphasized)
• Vou pagar amanhã pelo curso de português. (also possible, slightly different rhythm)

• amanhã = tomorrow
• de manhã = in the morning
  You can combine them if needed: Vou pagar amanhã de manhã. = I’ll pay tomorrow morning.

Accents in Portuguese can mark stress and/or distinguish sounds:

• português: the accent shows the stress is on the last syllable (-guês) and signals an ê sound.
• amanhã: -nh- represents a palatal nasal sound, and ã is a nasal vowel. The tilde (~) marks nasalization.

A rough, learner-friendly guide:

• Eu vou ≈ eh VOH (often eu is reduced in fast speech)
• pagar ≈ pah-GAR
• pelo ≈ PEH-loo
• curso ≈ KOOR-soo
• de português ≈ jee por-too-GEHSS (the de often sounds like dji/ji before p in many accents)
• amanhã ≈ ah-ma-NYAH (nasal ã at the end)