作為一個 adverb，"altogether" 的意思是「完全，總共，總之」：
My goals are altogether different from yours.
Altogether you owe me two thousand dollars.
The games were exciting, the music was great and the food was delicious. Altogether it was an awesome night!
上面例句中的 "altogether" 都不可以改成 "all together"，因為 "all together" 的意思是「一道，一起」：
It is time we confronted the enemy all together.
All together the soldiers march towards the barracks.
We went to his house all together.