Difference between 予定 and 計画

予定: Schedule; something that is decided in advance; what you think of doing whether you do or not.

計画: Plan; design; image you built concretely for action.

明日その計画を発表する予定だ。
I am scheduled to announced the plan tomorrow. (I am going to announce the plan tomorrow)

計画がだいなしになる: A plan comes to nothing
計画をねる: Consider a plan
計画的はんこう: Deliberate attack


予定どおりに着く: Arrive on time
しゅっさん予定日: Expected due date

Source: http://answers.yahoo.com/question/index?qid=20081127024654AAYkPuW

Some Phrases

What did you say?
何と言ったのですか。

Could you please repeat that?
もう一度言ってもらえますか。

Could you speak more slowly?
もう少しゆっくり話してもらえますか。

I don't understand what you said.
あなたの言ったことが分からない。

Source:
http://japanese-phrases.sakura.ne.jp/page16j.html

How to say 'I don't understand what you said'

Sorry, I don't understand what you just said. 
すみません、なんと仰ったのか分かりません。

仰る is the honorific form of 言う



I don't understand what you're saying.
りかいできません。Or
あなたが言っているをしりません。


Casually,
What are you saying?
何言ってるの？

I can't understand what you're saying.
何言ってるのか分からない。

Source:

http://answers.yahoo.com/question/index;_ylt=Avl0M2mKyRbAbfThxxEA7.Dsy6IX;_ylv=3?qid=20090216155007AAAwYz0

http://answers.yahoo.com/question/index?qid=20090211182718AA9abIC