Learn Italian: Considero questa offerta molto interessante. - I consider this offer very interesting.

Verbs of judging or evaluating, like considerare, ritenere, and giudicare, connect the direct object (questa offerta) straight to its descriptive complement (molto interessante). You don't need to insert essere (to be) or come (as) in between, even though we often add them in English.

Since 'offerta' is feminine, why doesn't 'interessante' end in -a?

Adjectives that end in -e in the dictionary, like interessante, are invariable for gender in the singular. They stay exactly the same whether they describe a feminine noun like offerta or a masculine noun like progetto. They only change to -i in the plural.

How could I use this sentence with the premise conjunctions we are currently learning?

This sentence is entirely in the indicative because it states a direct fact, making it a perfect main clause. If you introduce a premise before it, you will trigger the subjunctive in that dependent clause: Ammesso che sia tutto vero, considero questa offerta molto interessante (Assuming that it's all true, I consider this offer very interesting).

Could I use 'ritenere' instead of 'considerare' in this context?

Absolutely. Ritengo questa offerta molto interessante means exactly the same thing. At the B2 level, using ritenere (to believe/consider) is a great way to elevate your vocabulary and sound slightly more formal or professional.