Questions & Answers about Я думаю о соседях.
What case is the noun соседях in, and why?
The noun соседях is in the prepositional (locative) plural. In Russian, the preposition о (meaning “about” or “of”) always takes the prepositional case. Since сосед (neighbor) is plural here, you get о соседях.
Why does the preposition change from о to об sometimes, and not here?
Russian uses об instead of о only before words that begin with a vowel or a voiced h‑sound (e.g. об идее, об ошибке). Соседях starts with the consonant с, so we stay with о соседях.
Why is it соседях and not the nominative plural соседи?
After о, you must use the prepositional case, not the nominative. The nominative plural “neighbors” is соседи, but in the prepositional plural it becomes соседях.
What does the verb form думаю tell us about the subject and time?
Думаю is the first-person singular present tense of думать (to think). It literally means “I think” or “I am thinking.” Russian does not distinguish between simple present and present continuous—this one form covers both.