1. We need to cut 20 trees to make every 1 ton of paper cups.

1-2. We need to cut 20 trees to make every 1 t of paper cups.

2. We need 10 l of water to make a piece of A4 paper.

3. We need 10 liter of water to make a piece of A4 paper.

4. We need 10 liters of water to make a piece of A4 paper.
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Would you check the sentences? Which expressions do we have to use?

#1 - we spell out numerals below 11, so spell out "one"

#1-2 - We spell "ton," so one is correct in that regard.
#2 - again, I would spell out "liters."
#3 - Ten should be spelled out, and ten is more than one, so it should be "liters."
#4 - Spell out ten.

Note: If you abbreviate "ton" and "liter," it's not totally wrong, but "t" and "l" should have a period: "t."

Let's check the sentences and identify the correct expressions to use:

1. We need to cut 20 trees to make every 1 ton of paper cups.
- This sentence is correct. The expression "1 ton" is the appropriate way to express the unit of measurement for the amount of paper cups.

1-2. We need to cut 20 trees to make every 1 t of paper cups.
- This sentence is also correct. Using "1 t" as an abbreviation for "1 ton" is acceptable.

2. We need 10 l of water to make a piece of A4 paper.
- This sentence is correct. The expression "10 l" correctly represents 10 liters of water.

3. We need 10 liter of water to make a piece of A4 paper.
- This sentence is incorrect. The correct expression is "10 liters" since "liter" should be pluralized when specifying a quantity.

4. We need 10 liters of water to make a piece of A4 paper.
- This sentence is correct. "10 liters" is the correct expression to represent a quantity of water.

In summary, the correct expressions to use are "1 ton", "1 t", "10 liters", and "10 l" depending on the desired format or abbreviation.