An inflection expresses one or more grammatical categories with a prefix, suffix or infix, or another internal modification such as a vowel change. For example, the Latin verb ducam, meaning "I will lead", includes the suffix -am, expressing person (first), number (singular), and tense-mood (future indicative or present subjunctive). The use of this suffix is an inflection. In contrast, in the English clause "I will lead", the word lead is not inflected for any of person, number, or tense; it is simply the bare form of a verb.
The inflected form of a word often contains both one or more free morphemes (a unit of meaning which can stand by itself as a word), and one or more bound morphemes (a unit of meaning which cannot stand alone as a word). For example, the English word cars is a noun that is inflected for number, specifically to express the plural; the content morpheme car is unbound because it could stand alone as a word, while the suffix -s is bound because it cannot stand alone as a word. These two morphemes together form the inflected word cars.
Words that are never subject to inflection are said to be invariant; for example, the English verb must is an invariant item: it never takes a suffix or changes form to signify a different grammatical category. Its categories can be determined only from its context.
Requiring the forms or inflections of more than one word in a sentence to be compatible with each other according to the rules of the language is known as concord or agreement. For example, in "the choir sings", "choir" is a singular noun, so "sing" is constrained in the present tense to use the third person singular suffix "s".
In English most nouns are inflected for number with the inflectional plural affix-s (as in "dog" → "dog-s"), and most English verbs are inflected for tense with the inflectional past tense affix -ed (as in "call" → "call-ed"). English also inflects verbs by affixation to mark the third person singular in the present tense (with -s), and the present participle (with -ing). English short adjectives are inflected to mark comparative and superlative forms (with -er and -est respectively).
Despite the march towards regularization, modern English retains traces of its ancestry, with a minority of its words still using inflection by ablaut (sound change, mostly in verbs) and umlaut (a particular type of sound change, mostly in nouns), as well as long-short vowel alternation. For example:
Write, wrote, written (marking by ablaut variation, and also suffixing in the participle)