In grammar:

A synthetic grammatical system (relationships between the parts of the sentence were shown by the forms of the words rather than by their position or by auxiliary words). In the early periods of history the grammatical forms were built by means of: sound interchanges, inflections and suppletion.

Suppletion (inherited from Indo-European) – the usage of 2 or more different roots as forms of one and the same word:

Part of Speech	Indo-European Non-Germanic Languages		Germanic Languages
	Italian	русский	English	German
Personal Pronouns	io, mio, mi/me	я, меня, мне	I, my, mine, me	ich, mich, mir
Adjectives	buono, migliore, ottimo	хороший, лучше, лучший	good, better, best	gut, besser, bester
Some Verbs	essere, sono, e`, ero, saro`, etc.	есть, был, будет	be, is, are, am, was, were	sein, bin, ist, sind, war, gewesen, etc.

Inflections (inherited from Indo-European) – though in the Germanic languages inflections were simpler and shorter than in other Indo-European languages.

Let’s take the system of declensions as an example. In PG it was well-developed but in the Old Germanic languages, due to the stress that was fixed on the root and the weakening of the end of a word as a result, the declensions started to disappear. While the nouns and adjectives still preserved stem-suffixes, they had declensions but once the stem suffixes started to weaken and disappear, the declensions were lost as well and the endings were simplified and got fewer:

Word Structure
PG	mak-oj-an	root + stem-suffix(word-deriv.)	+ gram. ending(form-marker)
Old Germanic Languages	mac-ian	stem (root melted with stem-suffix)	+ gram. ending

Sound Interchange – the usage of interchange of vowels and consonants for the purpose of word- and form-building (e.g.: English: bear – birth, build – built, tooth – teeth; German: gebären – Geburt)

Ablaut/Vowel Gradation – an independent vowel interchange, unconnected with any phonetic conditions (phonetic environment/surrounding) used to differentiate between grammatical forms of one and the same word. The Germanic ablaut was consistently used in building the principle forms of strong verbs.

Jacob Grimm has subdivided all the verbs into two groups according to the way they build their principle forms:

		Strong Verbs (irregular)	Weak Verbs (regular)
		called so because they have preserved the richness of forms since the time of Proto-Germanic	called so because they have lost their old Proto-Germanic forms and acquired new ones
form-building		vowel interchange + gram. ending	suffix –d/t (a Germanic invention!!!)
E.g.	OE	reisan – rais – risum – risans	macian – macode - macod
			cepan – cepte - cept
	ModE	rise – rose - risen	make – made – made
			keep – kept – kept

The most important innovation in Gmc was the emergence of the new types of verbs – “weak”, past tense with the dental suffix d: open – opened, work – worked.