By adding a small "っ" (tsu) in front of a syllable, it causes the subsequent consonant to double. This kana results in a slight pause between the two consonants (a double consonant pause). A couple examples are かった (katta) and しっけ (shikke).
Certain sounds ending with -い(i) ( き(ki), し(shi), ち(chi), に(ni), ひ(hi), み(mi), り(ri) and their voiced variants) can be followed by small versions (ゃ, ゅ, and ょ) of the hiragana や(ya), ゆ(yu), and よ(yo). In this case, the two hiragana are not pronounced individually, but rather as one sound.
In most cases, the compound sound is the consonant of the base syllable followed by the modifier (fjord may be an example of a similar compound sound in English):
In other cases the y sound disapears entirely:
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