Rashi on Exodus 6:3/רש"י על שמות ו׳:ג׳

1וארא AND I APPEARED — to the patriarchs.

2באל שדי BY THE NAME OF GOD ALMIGHTY — I made certain promises to them and in the case of all of these I said unto them, “I am God Almighty”.

3ושמי ה׳ לא נודעתי להם BUT BY MY NAME THE LORD WAS I NOT KNOWN TO THEM — It is not written here לא הודעתי [My name the Lord] I did not make known to them, but לא נודעתי [by My name, the Lord], was I not known [unto them] — i. e. I was not recognised by them in My attribute of “keeping faith”, by reason of which My name is called ה׳, which denotes that I am certain to substantiate My promise, for, indeed, I made promises to them but did not fulfill them [during their lifetime].