<i>a) I should write it this way to keep it easy to read, <b>while to keep</b> it formal.</i><br/><br/>The above is clearly not grammatical. It needs to be this:<br/><br/><i>b) I should write it this way to keep it easy to read, <b>and </b>to keep it formal.</i><br/><br/>1) Why can't <b>'while'</b> join two infinitive phrases?<br/><br/> <br/><br/> <br/><br/>2) This one below seems right. Why can 'while' join an infintive to an ing (but not to another infinitive)?<br/><br/><i>c) I should write it this way to keep it easy to read, <b>while </b>keeping it formal.</i><br/><br/><i></i><br/><br/><i></i><br/><br/><i></i><br/><br/> Thanks