這是基於以下外國網友的簡單解答 (個人認為最簡明合理):
often:
for something that happens or one does frequently, but not on a regular basis.
We often go to this restaurant; they serve great food.
usually:
for something that happens or one does as a habit, so it is more regular and predictable(. 可預言的,可預料的)
He usually spends his vacation in Italy with his friends.
always:
for something that happens or one does in any case.
Whatever happens , he always arrives at time .
3)如不講 frequency 而講用法 ...
always 可以配合 continuous tense 來藉以表達對事情的不滿. 例如:
She always asks her teacher questions. (~ 她經常問老師問題)
She's always asking stupid questions. (~ 討厭她時常這樣做)
always (及 never) 又不只可用來表達客觀次數上的多少, 更表達一種最強的主觀意願, 因此我們會說:
I'llalways love you.
I'll never disappoint you. (though I sometimes make you unhappy)
We always do our bestto serve customers.
often 則較常配合 how, quite, very 來用 (例如: 問 how often; 不是 how usually / how always).
如果不是想強調某事經常發生, 而是在大多數情況之下, 較適宜用 usually, 例如: ...
She usually speaks very politely when she has to ask her teacher questions. (~ 不是強調她常問問題或常常有禮貌, 而是按你觀察, 每當她需要問問題時她都會裝得有禮貌)
Girls usually expect flowers and diamonds on Valentine's Day . (~ 不是說所有女仔時常都想著要人送禮, 是指一般而言, 大多數女性在情人節都如此期望)
4) every day/ week / month / year 的 frequency 次序十分明顯, 但 always 至 sometimes 等字是較含糊, 不能具體量化來數的.
如果你認同上述那位外國網友的解釋, 即 usually 的重點在於 regular, 則上述四個 every xxx, "較為" 適宜用 usually 來配合.
I'll always love you. I'll never disappoint you. ...
Just a promise! Do you really believe someone is so "perfect"?
But even if you "know" that it's not impossible, you still find it ok. That's why I said "always" is "subjective".
correction: I'll never disappoint you. (though I "know that" sometimes "I might" make you unhappy)
correction: But even if you "know" that it's [not possible], ...