In English, the suffix '-ed' is used to mark the past tense of regular verbs. In the word 'walked', the final sound is realized as /t/ due to phonological assimilation with the voiceless /k/ sound of the root. This morpheme serves the specific grammatical function of indicating past tense, making it a past tense morpheme.

The suffix '-s' in 'John's' is known as the possessive clitic or genitive marker. It indicates ownership or a specific relationship between the noun 'John' and the object that follows it. Unlike plural markers, this morpheme functions to denote possession, which is a distinct grammatical category in English morphology.

The source answer identifies this as an adjective forming morpheme, which is factually incorrect as the '-s' in 'boys' is a standard plural inflectional morpheme. This conflict arises because the provided answer key does not align with standard linguistic classification. The suffix '-s' here indicates plurality, not an adjective.

The suffix '-en' in 'strengthen' acts as a derivational morpheme that transforms the adjective 'strength' (or the noun base) into a verb. It functions to indicate the process of making something possess the quality of the root word. Therefore, it is classified as a verb-forming morpheme, as it changes the word class to a verb.

The source answer identifies these as allophones, but in linguistic morphology, these are technically allomorphs of the past tense morpheme '-ed'. Allomorphs are different phonetic realizations of the same morpheme based on the phonological environment. The variation between /t/, /d/, and /id/ is determined by the final sound of the root verb, which is a classic example of allomorphic variation.

In English grammar, the suffix '-s' (or '-es') added to a verb in the present tense indicates the third-person singular subject. In the phrase 'she sings', the subject 'she' is third-person singular, requiring the verb to be marked with this inflectional morpheme to ensure subject-verb agreement within the sentence structure.