Proto-Germanic possessed a small amount of "particles", which were uninflected words and affixes that did not serve a single syntactic function in a sentence, but were applied in a variety of cases. They are often considered adverbs, but since they could be used in combination with almost any word and in any place in the sentence, they are probably better considered particles or clitics.
Negation was expressed with the particle *ne (“not”). It was always placed before the word to be negated, which was usually the verb but could also be a noun, adjective or other kind of word. Since imperative forms of verbs were always placed first in the sentence, they could not be negated in this way. Instead, the subjunctive form was used and the negation particle was placed before it (which is seen in, for example, modern Spanish as well).
Negation could also be expressed using a prefix *un-, which fulfilled similar functions as it does in English today. It was normally attached to verbs and adjectives.
Affirmation was expressed with the particle *ja. By itself, it meant "yes", but it could also be used within a sentence to mean "so, such", as in archaic English yea high.
Sentence clitics were suffixes that were attached to the first word of the sentence or clause. More precisely, they always appeared as the second part of their clause. This rule was followed very strictly, and sentence clitics could even split a prefix from the word it was attached to (as is seen on several occasions in Gothic).
The particle *-hw expressed conjunction ("and"). In conjunctions of two nouns or adjectives, it was joined to the second word of the conjunction, just like Latin -que (of which it was a cognate). It could also be attached to a verb placed first in its clause, in which case it joined the entire clause to the previous one. The form of the suffix was normally *-hw. When it occurred after an obstruent (and possibly after other consonants as well) an epenthetic -u- was inserted, which caused the suffix to be delabialised through normal phonetic rules, and it became *-uh.
The suffix *-u was an interrogative particle and could best be translated as "whether" or "is it so that". It indicated yes/no questions or a choice on a certain option. It was attached to the word to which the question applied, and this word was always placed first in the sentence.
Proto-Germanic possessed a general prefix *ga- which could be used to indicate completeness in some way. This extended to a completed action, a collective group and so on. It was commonly attached to verbs and nouns.