I'm unsure how to properly use ga in these examples.
watashi no kodomo wa chuusha ga itakute, kowai to kikimashita.
tomodachi wa watashi no dannasan ga tanoshikute yasashii hito da to iimasu.
At first I thought ga was used because itakute was used in the first one. Am I right to assume because we already introduced my child and friend any subject (chuusha and dannasan)after that is marked with ga?
WA and GA are as always a bit much to grasp all at once. YesJapan materials are designed to teach you little by little so it isn't such a big task.
Your sentences are pretty long as examples and a few different rules can be considered.
One rule you learn early on is to never have WA twice in the same sentence. This is a great rule early on, but later on you learn that it isn't 100% true. Many grammar rules can be broken with the right circumstances!
In this sentence: Tomodachi wa watashi no dannasan ga tanoshikute yasashii hito da to iimasu. My friend says my husband is a fun and kind person.
The topic is your friend so TOMODACHI is marked with WA. There are 2 reasons GA can be used for the following portion.
1. DANNASAN is the subject so GA is used. 2. Unless you are specifically emphasizing or comparing something the adjectives are used with the particle GA.
In this sentence: Watashi no kodomo wa chuusha ga itakute, kowai to kikimashita. I heard that my kids are scared because getting shots hurt.
The thing that hurts is CHUUSHA and ITAI is an adjective so you use GA unless you are specifically emphasizing or comparing a CHUUSHA to something else, for example:
Chuusha ga itai kedo, kusuri WA itakunai desu. Shots hurt, but medicine does not.
There isn't an actual grammar rule that says you can't have WA twice in a sentence. But so many of the sentences you use the pattern is [topic] WA [subject] GA that it is safe to make a rule that says not to repeat WA twice. By the time you are saying things that use WA twice you generally are more advanced and will do it naturally.
I will end with this. The mean will not change if you use WA or GA mistakenly. The worst case scenario is that it sounds a bit weird. The great thing is, Japanese people almost always repeat back to you the correct version :-)
I don't want to drag this out but is there a specific rule for adjectives and ga? I know hoshii and itai use it but I went back to other lessons and I didn't see adjectives being used with it. Thanks for the help sensei.
"The great thing is, Japanese people almost always repeat back to you the correct version :-) "
that is so true! I have experienced it, and I was like:" umm ok.. I hope it did not sound like something bad, and that's why I was corrected..!" but now that you have mentioned it, I feel a bit at ease making mistakes i.e. trying to be fluent!