Questions & Answers about Myślę o czymś ważnym.
Why does the preposition o take the locative case here?
When the preposition o is used with verbs of thinking or speaking, like myśleć or mówić, it means 'about' and requires the locative case. If you used o with the accusative case instead, it would mean something closer to 'against' (like bumping into something).
Where does the word czymś come from?
It is the locative form of coś (something). To decline indefinite pronouns ending in -ś, you just decline the base word and stick -ś back on the end. Since the locative form of co (what) is czym, the locative of coś becomes czymś.
Why is the adjective ważnym in the neuter form?
Even though coś (something) doesn't look like a standard noun, it is treated grammatically as a neuter word. Because czymś is in the locative case here, the adjective describing it must take the neuter locative ending, changing ważny to ważnym.
Could I use the perfective verb pomyśleć instead of myśleć here?