Parshat Va`era begins (Shemot 6):
meforshim all address this issue, as it is an obvious one. One can make grammatical distinctions (e.g. Ibn Ezra), or talk about the difference between נוֹדַעְתִּי and hoda'ti, where the latter would have meant simply telling them the name (Rashi). I'm not going to go through all the various opinions here.
However, I would point out the approach that takes the making known of a name as not just informing someone that X is the name. Names of God represent ways in which He interacts with humanity. We see this almost overtly in the previous parsha, Shemot, in which Moshe's asks which name of God he should present to the Israelites. Shemot 3:13-15:
of the name, they did not experience the import of the name, whatever that import might be. Indeed, following this cryptic statement that He did not reveal to them YKVK, the next verse is how He will fulfill his covenant with them. Context strongly suggests that the name is not just a name but carries meaning of fulfilling His Promise.
Two specific explanations of the verse that I enjoyed, because I like reparsing of pesukim. See Tg. Yonatan:
While we have the etnachta right before the word shemi, Tg Yonatan appears to read the verse:
"and I appeared unto Abraham, unto Isaac, and unto Jacob, as God Almighty, and by My name YKVK. However, I made Me not known to them."
Thus, by both names does Hashem appear to the forefathers. The end of the verse states that he did not "make Himself known," which Tg Yonatan explains as that He did not appear face to face with them, panim el panim, as he is currently appearing to Moshe. Moshe, after all, was the greatest of the prophets.
Rashbam, also in the text pictured above, says something quite similar, if I read him right. (I am not so sure that I am.) Regarding וּשְׁמִי ה, he says vezeh kefel lashon. Thus, "I appeared to them as El Shaddai, and my primary name is YKVK." Still, I did not reveal myself to them by the primary name but rather by El Shaddai, where reveal means fulfilling the various promises. Thus ushemi is connected above but not below. He then channels Rashi, highlighting the difference between noda'ti and hoda'ti, and states that had it said hoda'ti, then shemi would connect to phrase that follows, but since it states noda'ti, is is connected to the phrase above.