1. The Gemara explains the difference between chapters of Tehilim that begin with the phrase "Mizmor l'David" and chapters that begin with "l'David Mizmor."
2. The Shechinah rests on a person when he is happy due to a Mitzvah.
3. The Gemara explains the difference in the tenses used in Kidush and in Shemoneh Esreh.
4. Rava notes that in Pumbedisa the blessings on Shabbos and Yom Tov were recited in the present tense.
5. The third cup at the Seder is used for the recitation of Birkas ha'Mazon.
A BIT MORE
1. The chapters that begin with "l'David Mizmor" were written after the Shechinah rested on him, causing him to burst out in song. The ones that start with "Mizmor l'David" were written when he burst out in song, and then afterwards the Shechinah rested on him.
2. The Gemara mentions Shaul ha'Melech as an example. He had someone play music for him to bring him to a state of happiness so that the Shechinah would rest on him.
3. In Shemoneh Esreh, we ask Hashem to sanctify us and redeem us in the present tense ("Kadsheinu b'Mitzvosecha," "Go'el Yisrael"), since we want Him to be merciful with us and sanctify and redeem us. When we recite Kidush or praise Hashem in Hallel, we use the past tense to praise Him for having sanctified us and redeemed us in the past.
4. In other words, the blessings of Kidush and Shemoneh Esreh end in "Mekadesh ha'Shabbos" or "Mekadesh Yisrael veha'Zemanim."
5. The fourth cup is used for the recitation of Hallel.