Contractions function almost identically to the full two-word phrase, but are only appropriate in some places in a sentence. It’s one of the weird quirks of this language we’ve.
This post needs some kind of warning sign.
I did not see that coming.
linguist here!! it’s less that contractions are only appropriate in some places in a sentence, and more that they’re only appropriate with some types of verb – mainly auxiliary verbs.
for example: “i have seen the dog” -> “i’ve seen the dog” is perfectly acceptable, because in this instance, the “have” is acting as an auxiliary verb. what it’s doing is taking the main verb, “see(n)”, and putting it in the past tense (along with some other tense stuff). it doesn’t mean “have” in the traditional sense of something like “to own”. you don’t own or have the dog, and you certainly don’t own the seeing of the dog, whatever that might mean – but you did see the dog, in the past. the “have” basically just modifies the meaning of “see(n)”. so, auxiliary “have” is contractible.
however, “there’s a dog i have sometimes” -> “there’s a dog i’ve sometimes” is unacceptable, because in this sentence the “have” is not functioning as an auxiliary verb that’s modifying another verb. there’s no other verb for it to modify! (yes, the “is” in “there’s” is something commonly considered a verb, but it’s not, especially in this particular cirumstance. just trust me on this one.) in this instance, the “have” is the main verb – and therefore actually an entirely different verb altogether. in the same way “flour” and “flower” sound the same, these two “have”s sound the same and are spelled the same, but mean different things and come from entirely different classes of word. you might not know this consciously, but as a native / fluent speaker, you still know it.
can you hear how, when you say “there’s a dog i’ve sometimes”, you’re waiting for something at the end? your brain expects there to be something else – another verb. “the dog i’ve sometimes what? sometimes hugged? sometimes played with?” you instinctively know that, for “have” to be used in the “-‘ve” contraction, it has to be an auxiliary verb – and therefore there should be a main verb following it, because auxiliaries need a main verb to modify.
this is why the contraction in just-shower-thoughts’ example sounds wrong – in their example, the have is the main verb, because there’s no other verb for it to modify. so, again, you get this strange feeling of the sentence being incomplete. your brain is waiting for a main verb to appear for the auxiliary to modify.
tl;dr: it’s not where in the sentence a “have” occurs that defines whether you can contract it, but what kind of a “have” it is. only auxiliary “have”s can be contracted.
(in some circumstances, you can seem to use main verb “have” in a contraction – “i have a dog and three horses” -> “i’ve a dog and three horses” is acceptable to some people, though it does have a definite non-standard, almost archaic feel to it. i suspect what’s happening here is that when you say this, you’re actually saying “i have got a dog and three horses” -> “i’ve
gota dog and three horses”, in which case the “have” is still an auxiliary modifying a silent “got”, instead of being the main verb, so it still works!)