Talking of proper nouns, how can we use them as common nouns?

Toyota is the name of a corporation or the founder of the corporation = proper noun

A Toyota (A member of a unique class of common nouns) =common noun

We can say his Toyota, many Toyotas ...etc

Agree 

Denis Popov

I think by making the proper nouns names for common things,

I don't think we can use them as common nouns in writing.

A proper noun always has its1st letter capitalized and it does not matter where this proper noun is (the beginning of the sentence, the middle of the sentence, or the end of the sentence).

Just use them the way you like