In what moment in the development of the German language were separable verbs introduced? Also, is there a linguistic reason behind their introduction? Thanks!
Separable verbs have actually not been "introduced" but actually have always been there so to speak.
Linguists posit that separable verbs are a primitive feature of Indo European languages and can even be found in non Indo-European languages (e.g. Hungarian).
You will find separable verbs in:
A. Modern Languages
Some present-day English phrasal verbs:
- To swear in: "The Prime Minister swore the whole cabinet in".
- To screw up: "He is always screwing things up".
Present-day German separable verbs.
- Abfahren: "Wann fährst du ab?"
Present-day Dutch separable verbs.
- Schoonmaken: "Ik maakte het huis schoon" (Ich mache das Haus schön/sauber).
Even Canadian French has been "contaminated" with preposition stranding due to a prolonged contact with English (but this is a recent phenomenon, just mentioned here for the record).
B. Ancient Languages
First there is the familiar Latin sentence structure where, as in German, infinitive verbs are thrown at the end of the sentence in LIFO order. Did Latin ever have separable verbs? As a matter of fact "Old Latin" and a few other ancient languages did. As I read in this article of which I reproduce here the relevant part.
- Many Modern [sic] Indo-European verbs are separable verbs, as in Homeric Greek, in Hittite1, in the oldest Vedic [...].
- Thus, in Latin the verb supplāktum, "beg humbly", "supplicate" (adj. supplāks, "suppliant", verb plākējō, "advise, persuade"), gives sup wos plākējō (cf. O.Lat. sub uos placō), "I entreat you", and not "*wos supplakējō", as Classic Lat. "uos supplicō".
- Non-personal forms, i.e. Nouns and Adjectives, form a compound (karmadharaya) with the preposition; as O.Ind. prasādaḥ, "favour", Lat subsidium, praesidium, O.Ind. apaciti, Gk. apotisis , “reprisal”, etc.
In Hittite at least the technical term is preverbs. Except for two of them, they are all separable. The same (directional) words can be used either as adverbs, prepositions or preverbs: anda (into), appa (away), arha (outward), katta (downward), para (forward), sara (upward). Looks familiar? Google "up" for "Hittite preverbs" for more.
Maybe the right question then could be "When and how did prepositions become [inseparable] parts of verbs?".
1 At that point, it is worth noting that Hittite is recognised as one of the oldest known Indo-European languages, if not the oldest — with such primitive characteristics as only two genders (animate/inanimate).
German has, by the way, some phrasal verbs as well in addition to the separable verbs. If you take for instance schauen (to look), you'll see that you can add prepositions like über or auf. In a sentence, these are inverted like separable verbs but are not quite the same in their infinitive form: über etwas schauen - ich schaue übers Meer. I'm not sure if this will make it easier for you to learn and understand separable verbs, but it's at least a common feature of both German and English