Breakdown of Мы шли через длинный тоннель, и нам мешал дым.
и
and
мы
we
идти
to go (on foot) / to proceed, go (of a process)
длинный
long
Questions & Answers about Мы шли через длинный тоннель, и нам мешал дым.
Why is тоннель in the accusative case here if it looks exactly like the dictionary form?
The preposition через (through/across) always requires the accusative case to show spatial crossing. Because тоннель (tunnel) is a masculine inanimate noun, its accusative form looks exactly the same as its nominative form. The same rule applies to the adjective длинный (long).
Could we use ходили instead of шли for 'were walking'?
Not in this context. The verb шли is unidirectional, showing that they were making a single, continuous trip in one direction through the tunnel. Using ходили would imply walking back and forth, or walking through the tunnel as a repeated habit.
Why does the second part say нам мешал дым instead of using мы (we)?
The verb мешать (to bother, hinder, or interfere) requires the person being bothered to be in the dative case. So 'we' becomes нам (to us). The smoke (дым) is the actual grammatical subject doing the bothering, which is why it stays in the nominative case.
Why does мешал have a masculine singular ending if it affects 'us'?
In Russian, the verb always matches the subject. The subject of the second clause is дым (smoke), which is a masculine noun. The verb does not agree with нам, because нам is simply an object in the dative case.