In Spanish, when the letter G precedes a U plus a hard vowel, the U and the vowel are both pronounced. The U is pronounced like an English W:
In order to obtain this W sound in front of a soft vowel, the Ü comes into play. The two dots over the U are called a dieresis and indicate that two adjacent vowels both need to be pronounced as a diphthong:
In Spanish, the dieresis is only found on the U, and it can only precede E or I. When a U is followed by a hard vowel, as in guapo, the W sound / diphthong is automatic. Remember that a U without dieresis + E or I just makes the G hard (lesson on hard/soft vowels); the dieresis is what indicates that the U has its own sound.
Germanic and Nordic languages have a similar-looking accent called an "umlaut."
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