Learn Hardcore Russian: На мокрой траве было холодно сидеть, поэтому мы перешли с поляны на сухой песок у воды. - It was cold to sit on the wet grass, so we moved from the clearing to the dry sand by the water.

Breakdown of На мокрой траве было холодно сидеть, поэтому мы перешли с поляны на сухой песок у воды.

Why is the first part было холодно сидеть instead of something like мы были холодны?

Because Russian often uses an impersonal construction to talk about conditions or how something feels.

было холодно сидеть = it was cold to sit / it was too cold to sit
There is no grammatical subject in this clause.
было is the past tense of быть, used here in a neutral, impersonal way.
холодно is not an adjective here; it is a predicative adverb/state word, meaning cold as a condition.
сидеть is the infinitive to sit and explains what was cold.

So the structure is basically:

This is a very common Russian pattern:

Why is it На мокрой траве? What case is траве?