Questions & Answers about Mein Großvater ist sehr alt.
Why doesn't the adjective alt have an ending here?
In German, adjectives only get an ending when they come directly before the noun they describe, like in ein alter Großvater. Because alt comes after the verb ist, it stays in its basic dictionary form.
Why is the word for my exactly Mein in this sentence?
Großvater is a masculine noun, and he is the subject of the sentence, which means he is in the nominative case. The masculine nominative form of the possessive pronoun is simply mein. You would use meinen if he were the direct object of a sentence.
How would I change alt if I wanted to say he is older?
To make the comparative form of alt, you add an umlaut and -er, making it älter. It is very common for short, single-syllable German adjectives to take an umlaut when comparing things.