Training particles 2

1) 저는 책을 쓰고 있어요.

What is being written? A book. "book" is the object of the verb. Therefore, use 을.

Your sentence was missing a verb. You could say "write" (써요) or "am writing" (쓰고 있어요).
2) 저는 커피를 마셔요.

"Coffee" was misspelled.

Even if that was the right particle, "coffee" ends in a vowel, so you would have used "는" anyway. However... what is being drank? Coffee. "coffee" is the object of the verb. Therefore, use 을.

3) 저는 바나나를 먹어요.

"eat" was misspelled.

What is being eaten? A banana. "banana" is the object of the verb. Therefore, use 를.

While "내가" isn't necessarily incorrect, "저는" is the better choice. The difference is the politeness and the particle. 저 is more polite than 나, so I would stick with using 저 for now. 는 is general, but 가 emphasises.

Here is an example to show the difference in meaning/nuance. I will use the name 현주.

현주는 바나나를 먹어요. Hyunju EATS BANANAS. (general statement)

현주가 바나나를 먹어요. HYUNJU eats banans. (describes Hyunju, but emphasises that HYUNJU and not someone else eats bananas; specific statement, like it's answering a question)

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Whatever receives the action is the object. I recommend doing exercises to identify the object of a verb in either Portuguese or English for practice. If you can understand the concept in your own language, it will be easier to apply what you know to a new language.

For example, identify the objects in these sentences:

1. I made a purse.

2. He sold the dog.

3. She fired the gun.

4. My grandmother drinks tea.

5. Mom never buys cupcakes.