Hi everybody

Could you please tell me why [is] this sentence
is right? because in my opinion it's not.
''He acted as if he were guilty.''
Isn't this the right one ?
''He acted as if he was guilty''
We are talking about the singular, right? >>HE
Then...shouldn't ''was'' be used instead of ''were''? 
Both sentences are fine, Carmenn. In English when we use conditional sentences to talk about doubtful or counterfactual things we often use the subjunctive form, which means [you likely know this already] that we combine 'were' with all subject pronouns
If I/you/he/she/we/they/it were ...
But the subjunctive is a moribund form in English. English has lost most subjunctive forms and for those that remain, we have at least one other form to describe the same thing.
So, that means that we can and do also use 'was' for some pronouns.
This,
''He acted as if he
were guilty.''
could be a clear counterfactual wherein the speaker is saying;
I know he isn't guilty but he acted like he was/is guilty.
OR it could be an expression of doubt wherein the speaker has no opinion on whether 'he' is guilty or not.
Remember that language has to describe a whole range of possibilities.