`友` can interpreted as `とも` or `ゆう` right? So in which word/sentence/context
do I use first and when do I pronounce it as another transcription?
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question
readings
When is 友 read as とも, and when is it read as ゆう?
It depends on context: what compound is the character involved in, if any,
what are the okurigana and so on.

友達：ともだち (tomodachi)

友人：ゆうじん (yuujin)

There are general rules, like lone kanji, or lone kanji word stems, being
kunyomi, and kanji compounds being onyomi. These rules are broken. Sometimes
kanji compounds have a kunyomi. Sometimes compounds are half onyomi, half
kunyomi.

The ultimate arbiter is the "master" assignment of spellings to the
vocabulary.

You just know that yuujin is a word, and tomohito isn't. Ah, but, there is a
Tomohito surname, and one way to write it is 友人. You would know that from
context, and other clues, like the presence of a given name, or a suffix like
さん, 様 (sama), 氏 (shi).
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answer