Plural vs singular?

Hello(or good night)!
I am confused because don’t know which form of verb to be used in the case of the sentence below:
“One should mention that exploratory wells is very expensive test and are used when non-related with drilling data have proven gas presence with high probability”
Please clarify it to me.

Maxim.

I think I have found the answer:
the idea is that noun that is subject affects on verb, not predicate.
the right sentence is
One should mention that exploratory wells ARE very expensive test and ARE used when non-related with drilling data have proven gas presence with high probability"

To help solve the awkwardness, merely set nouns in parallel. There are further problems with your sentence, and the phrase ‘when non-related with’ makes no sense.

One should mention that exploratory wells are very expensive tests and are used only when drilling data have indicated a high probability of the presence of gas.