Questions & Answers about Наш сосед долго был водителем.
Why is it водителем instead of the dictionary form водитель?
It is in the Instrumental case. In Russian, when you state what someone's profession was in the past using был (was), the profession takes the Instrumental case.
Why does the Instrumental form of водитель end in -ем rather than -ом?
Standard masculine nouns ending in a hard consonant take -ом (like студент becoming студентом). However, водитель ends in a soft sign (-ь), so it takes the soft Instrumental ending -ем.
Does using the Instrumental case here mean he isn't a driver anymore?
Yes, it strongly implies that! Using был plus the Instrumental case (был водителем) suggests a past phase of life or a temporary state. It tells us he is likely retired or working a different job now.
Why isn't there a word for "for" before долго?
In English we say "for a long time," but Russian expresses this entire idea with just the adverb долго (long). You rarely use a preposition in Russian to show how long an action lasted.