Do people actually use the word “kaputt” in conversation?
It is definitely the preferred word used by native speakers to say that something is broken in spoken language (both formal and informal). I think in 99% of all cases a native speaker would use the word "kaputt" to say that some thing is defect.
This is also true for informal written language (such as e-mail among friends). However, in formal written language (e.g. a letter to an insurance company) you would use other words.
In formal written language you would use "defekt" (which means: It does not work as expected) or you would say that something is "nicht in Ordnung" (which means: "not OK"). You would also tend to use expressions that describe more specifically how exactly something is "broken" - For example by writing that your car has problems with the engine instead of saying that your car is "broken".
mein Knie ist kaputt
For context, the injury is fixable but will likely require surgical repair.
It is as well used for health context. Although usually not in serious context:
Ich bin total kaputt. (To be exhausted)
Ich glaube mein Knie ist kaputt. (My knee hurts.)
I have already heard people saying that some parts of the body are "kaputt" really meaning that they are injured in a way that they cannot be healed any more.
However, normally you would use expressions that describe an injury more specifically when seriously speaking about health problems.
If the knee is not healthy due to a broken bone, for example, you'll say that the knee is "gebrochen". This word also translates to the word "broken" in English but it has another meaning:
- The word "kaputt" means "broken" with the meaning that something is not OK, does not work correctly etc...
- The word "gebrochen" (in this case) means that something (such as a bone) has broken into parts because of a strong force pressing/pushing against it
- When some chain, string, rope breaks because of pulling to strong, you'll use the word "gerissen"