Questions & Answers about Портной записал мерки.
Because it originally is an adjective-type form that is used as a noun. Russian has a number of words like this. Портной means tailor, but it declines like an adjective:
- nominative: портной
- genitive: портного
- dative: портному
- etc.
So even though it means a person and functions as a noun, its endings follow the adjective pattern.
It is in the nominative singular, because it is the subject of the sentence: the tailor is the one doing the action.
So in Портной записал мерки, the structure is:
- Портной = subject
- записал = verb
- мерки = direct object