German word order, a bit like German words themselves, are built like LEGO. The basic premise is that the sentence can be divided up into chunks, sometimes called ‘ideas’ or ‘positions’.
Here, the subject ‘ich’ is in ‘first position’, i.e. at the beginning of the sentence. The verb must always be in ‘second position’ in a sentence unless it is being sent to the end by something like a subordinating conjunction. Then we have the adverbial (3), the accusative object (4) and the past participle (5), which almost always goes at the end of the sentence. We sometimes also call the verb positions 2 and 5 the ‘verb brackets’ because they are the two positions where you can have a verb, and in situations like a past tense sentence they tend to ‘bracket’ the rest of the text. The more stuff we have in the sentence, the more ‘ideas’ we have, so these numbers won’t always apply to the same element; therefore the most important things to remember are first position, second position, the ‘verb brackets’, and the fact that the accusative object always comes after the bulk of the adverbials.
Here, we’ve put ‘Im Fitnessstudio’ in first position, either because we want to emphasise that we lost our Handy in the gym or because we’re just mixing our word order up a bit for effect. First position is now full, and the verb must be in second position, so now we have to put the subject in the earliest position available, which is third. This is called ‘subject-verb inversion’ because we have basically swapped around the
Now we’ve put a time phrase in first position so we can tell our angry dad that we lost our phone yesterday. As always, the verb has to be in second position, and the subject has to come as early as possible so it’s in third position again. The adjectival comes next because the accusative object has to come after the adjectivals, and then we have the second verb bracket.