1. There were two seeds.

The first seed was negative, but the second seed was positive.

2. There were two seeds.
One seed was negative, but the other seed was positive.

3. There were two seeds.
The first seed was negative, but the other seed was positive.

4. There were two seeds.
One seed was negative, but the second seed was positive.

(Which expressions are corrct? And then, would you let me know the reason?)

Sentences 1 and 2 are correct because they are balanced. Do you see where the imbalance is in 3. and 4.?

All four expressions are correct, but they have slightly different meanings and emphasize different information. Let's break them down:

1. "The first seed was negative, but the second seed was positive."
This expression explicitly states that the first seed was negative, and it emphasizes the order in which the seeds were mentioned. It implies that the order of mentioning the seeds is important for understanding the context.

2. "One seed was negative, but the other seed was positive."
This expression does not specify the order of the seeds or which one is being referred to as the first or second seed. It simply states that there were two seeds, one of which was negative and the other positive. This expression is useful when the order of mentioning the seeds is not important or unknown.

3. "The first seed was negative, but the other seed was positive."
This expression is similar to the first one, but it uses the term "the other seed" instead of "the second seed." It implies that there are only two seeds and that the second seed is being referred to as "the other." It also emphasizes the contrast between the negative first seed and the positive other seed.

4. "One seed was negative, but the second seed was positive."
This expression is similar to the second one, but it explicitly mentions the second seed as "the second seed." It implies that there are only two seeds, and it specifies the order in which the seeds are presented. It can be useful when emphasizing the chronological or sequential nature of the information.

The choice of expression depends on the specific context and the intended emphasis or clarity of the information being conveyed.