Assimilation is when two sounds come together and change or melt into a new sound. Assimilations may happen inside a word, or between two words, when the final sound of a word touches the first sound of the next word (because when we speak we join all the words together). When speaking, people make many assimilations, most of them can only be noticed by the trained ear of an academic speaker and are not important for us. But there are 4 assimilations that are very important, because the sound changes a lot and the student of English can notice the difference and feel confused about it.

The sound /j/ is usually found in the letter Y, the letter I and the letter U (when it is or was in the past pronounced /ju:/)

1- /t/ + /j/ = /tʃ/  (T+Y=CH)

Nice to meet you meet + you /mi:t  ju:/ = /mi:tʃu:/
picture 1 /pɪktjʊə*/ → 2 /pɪktʃʊə*/ → 3 /pɪktʃə*/  (1= old fashioned pronunciation, 2= transitional pronunciation, 3= present pronunciation, the vowel /ʊ/ weakened and disappears)

2- /d/ + /j/ = /dʒ/ (D+Y=J)

would you like some tea? would + you /wʊd ju:/ = /wʊdʒu:/
soldier  /səʊldjə*/ = /səʊldʒə*/

3- /s/ + /j/ = /ʃ/ (S+Y=SH)

special   /spesjəl/ = /speʃəl/
it's just you = it's jus' you /dʒʌs  ju:/ = /dʒʌʃu:/ (in conversational English "just" often loses its final T and it sounds "jus", so S + Y = SH)

4- /z/ + /j/ = /ʒ/

treasure /trezjʊə*/ → 2 /treʒʊə*/ → 3 /treʒə*/ (1, 2, 3= see comments to the word "picture" above)
learning centres such as universities as + universities /əz  ju:nɪvɜ:*sɪtɪz/ = /əʒu:nɪvɜ:*sɪtɪz/