Contact
Every message is read by a person. Which route to use depends on what went wrong: a question in a lesson, or anything else.
A question is wrong
Use the report control on the question itself. Choose Report a problem, then say what you expected. The report keeps the question, the answer you gave, the answer Sensim expected and how your answer was read, so the problem can be replayed exactly.
That is the right route for a wrong expected answer, a question that cannot be typed into the box, a figure that does not load or a mistake in an explanation.
Leave a message
For anything else (your account, a page that breaks, an idea) write it here. Say which page you were on and what you saw.
What not to send
Never send your password to anyone. Nothing in Sensim will ask for it.