Ja dodaję cukier, żeby herbata była słodka, nie gorzka.

Polish verbs come in two aspects:

• Imperfective (e.g. dodaję) describes ongoing, habitual, or repeated actions (“I add” / “I am adding”).
• Perfective (e.g. dodam) describes a single, completed action, often in the future (“I will add (once)”).

Since the speaker is talking about a general habit or current action, the imperfective dodaję is the correct choice.

Both żeby and aby mean “so that” or “in order to.” Differences:

• Aby is slightly more formal or literary.
• Żeby is the everyday spoken form and actually derives from że + by.

In most casual contexts and conversations, żeby is preferred for introducing purpose clauses.

Polish doesn’t have a separate subjunctive mood. To express a desired or hypothetical state, you use:

1. The past-tense form of the verb (here była, feminine form of “was”)
2. Plus the particle by, which is already built into żeby

So żeby herbata była słodka literally means “so that the tea would be sweet.” Using jest would just state a fact: “the tea is sweet.”

In Polish, adjectives must agree with the noun’s gender, number, and case. Here:

• Herbata is feminine singular in the nominative case.
• Therefore adjectives describing it take the feminine singular nominative ending –a: słodka, gorzka.

This is ellipsis in action:

• The verb and subject (herbata była) from the first clause carry over.
• We contrast słodka with nie gorzka without repeating herbata była.

If you want to make the contrast more explicit, you can insert a:
…żeby herbata była słodka, a nie gorzka.
But dropping a is perfectly natural and common.

Absolutely. Polish often omits pronouns and even nouns when the context is clear. For example:

• Dodaję cukier, żeby była słodka, nie gorzka.

Here, both ja and the second herbata are left out, but the meaning remains clear because of verb endings and context. Including them is a stylistic choice for emphasis or clarity.