Hey, <br /><br />you noticed the sound-changes very well. There are entire books about it. You can also see the changes from old-Latin to Classical Latin and Medieval Latin. Most of the sound changes you can check in a historical grammar, like Niederman, Phonétique historique du Latin. Or in English: Palmer, The Latin language. The first is an entire book about it. <br />If you only want to see the changes in classical Latin, you can also find them sometimes in regular grammars. <br /><br />With 'abesse' you have absum, afui, (afuturus)<br /> 'adesse' you have adsum, sometimes assum, adfui and sometimes affui. <br />Obesse = obsum, obfui <br /><br />In Rome with most words you had two manners of orthographe; a conservative and a progressive. The conservative was the etymological one and in the progressive way they wrote nearly like they spoke. <br />There was always one orthography the most cummon one.<br />Besides obtineo, sometimes we also find optineo, cause that was how they pronounced it! <br />If you really want to be sure, you can always see in a dictionnary or in one of the books mentioned above. <br /><br />Greetz, <br />Moerus.