The Quick Answer
Fourth relates to the number four (e.g., fourth place).
Fourth與數字四有關(比方說，第四位)。
Forth usually means forward or onward.
Forth的意思是向前進或者向前推進。
Forth
The word forth (without u) is an adverb meaning forward or onward (e.g., from this day forth, bring forth, go forth). It can also mean come out into view (e.g., Come forth from the crowd).
forth(沒有u)是一個副詞，意思是向前進(比方說，從今天起，產生，向前去)。它也可以表示從視野出來(從人群中出來)。
The popular phrase back and forth makes use of the word forth.
短語back and forth(反覆地，來回地)就是使用了forth這個單詞。
Examples:
Put a cherry on top the first cake, then the second, and so forth.
(and so forth= more of the same)
將櫻桃放在第一塊蛋糕上，然後放在第二塊蛋糕上，依此類推。
(and so forth=更多和這個一樣的，以此類推、諸如此類)
I need to be present at the meeting to put my ideas forth.
我需要出席會議來提出我的想法。
Fourth
The word fourth (with u) relates to the number four. It can be a noun (e.g., one fourth, i.e., a quarter), an adjective (e.g., the fourth car) or an adverb (e.g., the new driver came fourth).
Fourth(有u)與數字四相關。它可以是一個名詞(比方說，四分之一)，或者是形容詞(比方說，第四輛車)或者是副詞(比方說，這新車手排名第四)。
很多同學會把這兩個詞記混是因為他們總覺得這是一個單詞的兩種寫法，因為在英式英語和美式英語中經常會有多一個u或少一個u的拼寫差異，但這兩個詞並不是這樣的。其實，你只要記住fourth與four都是數詞，而forth是“前進地”的意思就可以啦。
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