Deutsch Als Fremdsprache

I've never thought about this but I'd tentatively say it's when there's an "e" at the end of the preceding word. It doesn't feel natural to skip the "e" in "es". Saying "hasste's" is also ambiguous, are you saying "hasste es" or "hasst es"?

Well, I'm a native speaker not a language teacher, so I'm not 100% sure, but I think part of the reason that you can't leave out the e in "es" here is that "hassen" (to hate) ends with an "e". All the examples where you can shorten "es" that I can come up with right now end with an consonant.

In any case, you can ignore the option to shorten "es" in vernacular German too. It's not as ubiquitous as the English shortenings of verbs like "is" and "are". So if you're unsure do that.

I don't know about the grammar but will give my 2 cents regardless.

I just went through some examples I could come up with in my head and would only use it in spoken German mostly in Präsens or Perfekt tense (which is mostly due to those 2 being used in spoken German the most I would presume). But also in Futur 1 and 2. Same for informal chats on Whatsapp.

Usually I use it with general statements about a subject in Präsens or as a general reduction that is almost always acceptable to me to shorten the sentence in Perfekt

I don't use the apostrophe personally unless Autocorrect puts it in there for me.

Präsens:

• Er hassts wenn man das macht.
• Sie liebts wenn man das macht.
• Sie habens nicht richtig gemacht. (Plural or formal)

Perfekt:

• Das Tier hat's gefressen.
• Ich hab mein Geld am Automaten vergessen. Der Automat hat's einfach wieder eingezogen. Notice the reduction of "habe" as well into "hab" in the first sentence of the second example.

Futur 1: Ich werd's machen

Futur 2: Ich werd's dann vorbereitet haben.

The example you gave is in Plusquamperfekt and I would generally not use the reduction there. Same for Präteritum.

Hope this helps.

Thanks all here for the insights!