You do not need certainty to contribute usefully to a conversation. Imagine someone asks when a community class begins. You remember an earlier timetable but have not checked the current one. Say, "Last term it began at six. I have not checked this term's time."
This is more informative than either "Six o'clock" stated confidently or "I do not know" with no context. The listener learns which part is remembered and which part needs confirmation.
Choose an expression that matches your evidence
"I think" signals a belief, but it does not explain its basis. Add the reason when it matters: "I think the office is closed because the notice says it closes at four." If you only have a guess, identify it as such.
Offer a checking route: "The booking confirmation should have the current start time" or "We can ask the organizer." Do not promise to check later unless you genuinely intend to do so.
Practice three versions of the same answer: a confirmed fact, a remembered detail, and a possibility. Notice how words such as "might," "probably," and "according to the message" change the listener's expectations. For decisions involving travel or deadlines, make the unconfirmed detail especially visible so the other person knows to verify it before acting.