1. Stop at the red light.

2. Stop on the red light.
(Which preposition should we use?)

3. Turn slightly right.
4. This is a one-way street.
(Are both grammatical?)

5. Fill it up with regular unleaded, please.
6. Fill her up with regular, please.
7. Fill it up with the unleaded.
(Are the three the same? Which one is commonly used? Thank you.)
English - Writeacher, Friday, April 4, 2014 at 4:05pm
#s 1, 3, 4, 5, and 7 are correct and commonly used.

#6 is used in a slangy way in some parts of the US!

If you use #2, people will understand you, but it's not used much at all.
English - Ms. Sue, Friday, April 4, 2014 at 4:29pm
Also -- I don't know of any gas stations that have attendants. All the stations I've seen are self-serve -- so 5-7 are useless.
English - Writeacher, Friday, April 4, 2014 at 4:37pm
There might be a few full-service stations left in Korea!

................
English - Ms. Sue, Friday, April 4, 2014 at 5:07pm
:-)
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Thank you for your help.

1. Stop at the red light.
2. Stop on the red light.
(Then, is #2 grammatical wrong/incorrect? If somebody writes "Stop on the red light," is it wrong in composition/writing?

Yes, you should say/write, "Stop at the red light." Don't use "on" for this sentence.

Yes, "Stop on the red light" is grammatically incorrect. The correct preposition to use in this context is "at." So, if somebody writes "Stop on the red light," it would be considered grammatically wrong or incorrect. The correct sentence would be "Stop at the red light."