you mean when NEN + CHUU = NENJUU right? in this case, the ち in ちゅう changed from unvoiced consonant to VOICED consonant, that is your vocal chord vibrating (voiced) or not (for 'chuu' it's not precisely that but that's the idea, it's not entirely true because japanese pronunciation changed over time but for now, pls just leave it at that xd). other example of this phenomenon is 中国 Koku -> Goku, 人数 Suu -> Zuu 青空 ao + sora = AoZora and so on. In these examples, when you finish pronounce the first word, your vocal chord is vibrating to pronounce the vowel, so it's convenient to just let it keep vibrating while moving on to the 2nd word rather than turning it off to pronounce an unvoiced consonant then turn it on again. Try to pronounce Nenchuu and Nenjuu, you can feel that when pronouncing Nenchuu, your throat stops vibrating for a split second to pronounce CH while Nenjuu keeps your throat vibrate all the way. Someone proposed a method to identify which combination of word would require you to change pronunciation, but it's not proven yet (or pretty difficult to do unless you are a trained linguist) so for the mean time, it's best just to check.
The reason it sounds differently when combined, is because in this way, it makes the Next sound/consonant easier/faster to pronounce. It concerns the movement of your mouth/vocal chords, so as a beginner learner, you probably wont feel the ease/difference but natives would (sometimes depends on their accent and dialect though).
In English, words like National and Nationality are pronounced differently (Again, probably because it's easier). In french, when "de" + "le", it becomes "du" probably because they have the same vowels anyway. Their exact circumstances are different but generally still follow the logic of "ease" to pronounce.
How kanji is combined here?
Hello
I wanted to connect two kanjis (year and middle) to make another word , but it sounds different than the kanjis separately. How does it work? I mean how combined sounds another way?
Thank you