I saw a magnificent picture.

To translate this sentence, we must first understand its sentence components.

I saw a magnificent picture. The subject of this sentence is I (I), the predicate is saw (past tense), and the object is picture (picture), granular (magnificent; Impressed and excellent; This is an attribute, adjectives modify nouns and pictures, so it is a magnificent picture.

I saw a magnificent picture in the past.

More generally, I have seen a magnificent photo before.