hoe gaat het, meneer ferry? welcome to the wonderful world of japanese! that "は" is such an important article of speech. you will see it in the vast majority of sentences, so getting used to it now is really great advice your teacher gave!

just wanted to let you know the phrase is spelled "こんにち"　(今日) not "こんいち"　(？懇意地？). even though in english and dutch 2 N's make the same sound as 1 N, to the japanese it can form completely different words. this will become apparent when you start learning kanji (as you can see!) and will be forced to differentiate between on-yomi and kun-yomi. veel succes!

Hi, thank you for you friend request and message. Hope to help your learning Japanese and practicing Dutch together.

I would say you can use wa/ha in writing with limitaion.

You can never use wa(which means こんにちわ) in academic and formal writing, so please be sure you use this expression only when you text with your friends or something informal situation.