Questions & Answers about Они нашли старый дневник, который мы здесь храним.
Why doesn't который change its ending here if it's the direct object of 'we keep'?
Который replaces дневник (diary), which is a masculine inanimate noun. In the second part of the sentence ("we keep which here"), it acts as the direct object, so it must be in the Accusative case. For masculine inanimate words, the Accusative form is identical to the Nominative form, which is why it stays который.
Is старый дневник in the Nominative or Accusative case here?
It is in the Accusative case because it's the direct object of нашли (they found). Just like the pronoun который, because дневник is a masculine inanimate noun, its Accusative form looks exactly like its dictionary (Nominative) form.
Why is there a comma before который?
In Russian, you must always put a comma before a relative clause starting with words like который (which/who). In English, we often write "the diary which we keep" without a comma, but Russian rigidly requires it to separate the two parts of the sentence.
What is the dictionary (infinitive) form of the verb нашли?
The infinitive is найти (to find). It is a perfective verb with slightly irregular past tense forms that drop the "т": нашёл (he found), нашла (she found), нашло (it found), and нашли (they found).