Learn Croatian: Ovo je lažna torba, ali logo je isti. - This is a fake bag, but the logo is the same.

The word isti (the same) acts like an adjective in Croatian, meaning it must agree in gender, number, and case with the noun it describes. Here, it describes logo, which is a masculine singular noun in the nominative case (it is the subject of the second half of the sentence). Therefore, we use the masculine nominative singular form isti.

Many Croatian adjectives have both an indefinite (short) and a definite (long) form. Does isti have a short form?

No, isti only exists in the definite form, which is why it ends in -i for the masculine nominative. Because the concept of "the same" inherently points to something specific and defined, it never uses an indefinite ending. You will never see "ist" used as a standalone masculine adjective.

torba (bag) is a feminine noun. Why does the sentence start with the neuter pronoun ovo instead of the feminine ova?

When you are introducing or pointing something out using the structure "This is a [noun]", you always use the fixed neuter pronoun ovo (or to for "that is"), regardless of the gender of the noun that follows. You would only use the feminine ova if the pronoun directly attached to the noun as an adjective, like in the phrase ova torba je lažna (this bag is fake).

The word logo ends in "-o". Doesn't that usually mean it's a neuter noun in Croatian?