Singular or plural

[diplomacy]

Since I knew that:six minute,ten dollars,and common errors are singular,I have been confused.
They seem as plural,but indeed,they are singular.
Are there any rules for that?

By the way,are these sentences correct?

1/ six feet aren't enough for the measurement.
2/ players are playing nice now.
3/ four shops don't fit to assurance of quality standards.

[5jj]

Since I knew that:six minute,ten dollars,and common errors are singular,I have been confused. They seem as plural,but indeed,they are singular.
Are there any rules for that?

When we think of these as amounts of time/money/etc, they are singular: Six minutes is all I need. Ten dollars is all I have.

By the way, are these sentences correct?

1/ six feet aren't enough for the measurement.
2/ players are playing nice now.
3/ four shops don't fit to assurance of quality standards.

I am not sure what message you intend to convey with these unnatural sentences.

[diplomacy]

It means if we are talking about things in amount we should consider them as singular otherwise,they are plural.
Last three sentences don't work with this rule?

[Barb_D]

I"m afraid the last three sentences aren't very understandable.

If you think of something (time, distance, money) as a whole "mass" then you can use the singular.

My sister runs marathons. For me, 26 miles is too far to run. She says the first 20 (individual miles) aren't bad, but the last six miles are. By the time she is at the 20-mile point, she is aware of each additional mile. So "20 miles is easy, but the last six miles of a marathon are hard."