The site owner (and site administrators, if allowed) can use the proxy mechanism to log in to any learner's (or other site administrator's) account. The proxy user (site owner / admin) would use his/her own authentication credentials for this purpose and therefore wont use (and need not know the credentials of) the account to which he/she would login. To use this feature, use the menu item Users -> Proxy Login.
This menu item would be visible to the site owner and, if desired, can be made visible to all site administrators through the option in the Site Admin -> Site Details -> Security. This feature would not be available to the learner role.
The application would prompt the user to confirm that he/she would be logged out of the current session and would then be presented with the login page of the application.
The URL of the page consists of an additional parameter ?proxy=true (this must be present in order for proxy feature to work, if you reload the signin page from any other way, and this parameter is not there in URL, you wouldn't be able to use proxy... you can notice additional help message below username field when using proxy parameter)
Specify the username in the format of adminusername/someotherusername (admin user name followed by forward slash followed by any other user name). Specify your own password in the password field. You would now be logged in as the other user. Note that this would log out the other user from any currently active session (if any).