Most European languages use grammatical prefixes and/or suffixes to indicate what word class it belongs to.
For example, the German for the verb "to love" is "lieben" -- the "-en" ending indicates the infinite. The noun, though, is "die Liebe", which has a different ending and (because this is German) a capital letter.
In Russian it's much the same. The verb is "lyubit'" -- the t' ending is a good indicator that it is a verb. The noun however is "lyubov'", which again has a different ending.
English does still use grammatical endings, but not as often. English also uses sentence structure and word order.
In the sentence "I love you", you have a subject pronoun, followed by a word, followed by an object pronoun. That second word must be a verb; there's nothing else that can stand alone between a subject and an object in a simple sentence like this.
In the sentence "Love is a wonderful thing", we have a cupola "is" which links a subject and a complement. The complement is "a wonderful thing", and the subject is "love". A verb cannot be a subject (unless you turn it into a gerund -- "loving"), so it must be a noun here.
And that's all there is to it. Other languages use morphology, English uses a mixture of morphology and sentence structure.
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