This is the 17th most frequent Polish word.
tego
"Tego" means "of this" or "this" (genitive or accusative singular form of "ten").
Here 'tego' is used as a pronoun referring to the object of learning in the genitive case.
Powinieneś się tego nauczyć.
You should learn this.
In this sentence, 'tego' is used to specify the object of the speaker's uncertainty.
Nie jestem tego pewien.
I am not sure about this.
The word 'tego' refers to a specific noun (car) in the genitive case.