I don't understand what is the difference between these two:<br/><br/> (1) This class is unavailable to students.<br/><br/> (2) This class is unavailable for students.<br/><br/>If I write those sentences like this, does the meaning change? :<br/><br/>(3) At the moment, this class is unavailable to students.<br/><br/> (4) At the moment, this class is unavailable for students.<br/><br/>---<br/><br/>Are these correct?<br/><br/> I got here by plane.<br/><br/> I got here by walking.<br/><br/> I got here by feet. <br/><br/> I got here on feet. - Does this mean that I arrived safely and that I'm not sick?