1)
Why did he say "Hashem Sefasai Tiftach [u'Fi Yagid Tehilasecha]"?
1.
Rashi: Pardon me, and I will properly be able to speak Your praise.
2.
Radak: When my tongue will sing Your virtue, return to me Ru'ach ha'Kodesh that will appear on my tongue, like I used to have.
3.
Toras Moshe (Devarim 3:23): Even if his Tefilah is fluent in his mouth, due to the awesome fear of standing in front of Hashem, the King of kings, his knees will knock and he will be unable to stand.
4.
Malbim: My lips and mouth, i.e. external speech, will inform others of Your praise.
