Yes, it is possible for someone to use your internet without knowing your password, although modern security measures make this less common if proper precautions are taken.
Unsecured Network: If your Wi-Fi network has no password protection, anyone within range can easily connect to it.
Giving out your password: Sharing your Wi-Fi password gives guests full access to your local network, creating risks like data exposure, bandwidth hogging, and potential malware spread.
Hacking/Cracking the Password: Attackers with advanced skills and specific tools can attempt to crack weak passwords, although modern WPA2 and WPA3 encryption makes this difficult if a strong, unique password (a "passphrase") is used.
Malware: If a device already connected to your network is infected with malware, an attacker might gain remote access to your network through that compromised device.
Physical Access: Someone with physical access to your router can easily connect a device via an Ethernet cable or use the WPS button, bypassing the need to know the wireless password.