"Arigatou" means "thank you" in Japanese. "Arigatou" is coming from the word "arigatai" "Arigatai" is used to explain the things mearly happened. Since my English is not good, I'm not sure I can explain what you need.

Probably, maybe.

I think 結構です is polite enough to say "no" in Japanease . If you want to be more polite, then you can add "お気持ちはありがたいのですが" before "結構です" "お気持ちはありがたいのですが" means "I appriciare your concern but....."

科学の目的

この研究室での私たちの目的は、天秤を使わずに、モーターに取り付けられた対象の質量をはかることです。

How come you can do it?



Scientific Objective
Our objective in this lab is to find the mass of an object that is attached to a motor without using a scale.

I think you can use both が and は in this sentence. When, I hear 私は犬はすきじゃない , I assume you might like the other aimals. When you use が, the sentence dosen't have such nuance. It's only my opinion.

出=out 手=hand 手を洗ってから食べてください 食べる前に手を洗ってください Both are correct. Also, 食べはじめる前に手を洗ってください is correct. When たべる is conjunctioned with はじめる, る is omitted.

2)予習　is preparation for future class or examination . 予 means beforehand and 習 means learn. 準備=用意