The genitive and dative of 1st declension nouns are always circumflex (perispomenon), IF accented on the ultima, and never accute; the nominative singular and accusative singular, if η and if accented, is always accute (oxytone). Note that 1st declension genitive plurals are always accented on the ultima, unlike the other declensions. (Well, I cannot get cut text from Logos' Smyth or from the UChicago Smyth to appear correctly, so here is a snapshot of the relevant sections.) You can find the UofChicago Smyth at http://artflx.uchicago.edu/cgi-bin/philologic/getobject.pl?c.9:3:0.NewPerseusMonographs
. Smyth, H. W. (1920). A Greek Grammar for Colleges (48–49). New York; Cincinnati; Chicago; Boston; Atlanta: American Book Company. You can get a copy of Smyth for about $45US. See the thread about Smyth's grammar at http://www.ibiblio.org/bgreek/forum/viewtopic.php?f=21&t=1020&hilit=smyth
. I think the real question is why the nominative and accusative take the accute accent. I'm going to post this and let others comment. I don't want to lose the inline images.