There's only one letter that is different between these two words, but they mean completely different things.
These words are spelled exactly the same except for one letter, but they are completely different in meaning.
ーThere's only one letter that is different between these two words, but they mean completely different things.
「この２つの単語にはたった一文字の違いしかないけど、完全に異なることを意味します。」
they mean completely different things で「それらは（２つの単語）完全に異なることを意味する」
ーThese words are spelled exactly the same except for one letter, but they are completely different in meaning.
「この２つの単語は一文字を除いて全く同じですが、意味が全く異なります。」
be spelled で「綴られる」
exactly the same で「全く同じ」
except で「〜を除いては」
different in meaning で「意味が違う」
ご参考まで！
A difference of just one letter can make the meaning so different.
A single spelling difference can entirely change the word's meaning.
ご質問ありがとうございます。
①
"A difference of just one letter"＝「一文字の違い」
"can make the meaning so different."＝「によって意味がこんなに違うとは。」
②
"A single spelling difference"＝「一つのスペルの違い」
"can entirely change the word's meaning."=「は言葉の意味を完全に変えることができるんだね。」
ご参考になると良いです！
カナダ 
アメリカ合衆国 
