he always carried the umbrella when he want out點解要用過去式?

Is it the original sentence is "he always carried the umbrella when he went out"?

Have a look at the sentence , both the word " carried" and " went" are past tense.

That means it is a past habit (過去習慣) for the man to carry the umberlla .( In the past )when he went out , he always carried the umberlla .

So if you want to show the present habit of that man, you may say,
"He always carries the umbrella when he goes out"

因為他說[常常]

因為已經係以前既事,所以要用過去式