1)
Why does it say "Al Tis'halel b'Yom Machar"?
1.
Bereishis Rabah 73:9: Hashem criticized Yakov for saying "v'Ansah Bi Tzidkasi b'Yom Machar" (Bereishis 30:33) - later, your daughter will go out and be molested!
2.
Rashi: Do not glorify yourself now about something that will be in the future.
3.
Malbim: Mishaps of time occur constantly, so you cannot boast about what you will do in the future.
2)
What do we learn from "Ki Lo Seda Mah Yeled Yom"?
1.
Rashi: Perhaps today some evil will arise that will nullify intents for the future.
2.
Malbim: Even what will happen today, you do not know, and tomorrow is not yours (in your control).
