rendaku and sokuon

[weal53005213]

how I can understand to use rendaku and sokuon

[Psy]

The simple solution is to learn a lot of vocabulary and notice the patterns. Things like 発信 はっしん happen when you merge はつ and しん, often with words of Chinese origin. Rendaku happens a lot with Japanese word roots, e.g. 昔話 むかしばなし from むかし and はなし. There are a ton of examples out there, but this is one of those things that you'll pick up on very easily with simple exposure to the language.

In other words, don't stress out over it.