The consonant at the end of a word carries the grammar
Plural, past and third person all live in the final consonant, so dropping it drops the grammar.
My friend works in two shops and she helped me last week.
English words very often end in a consonant, and a great deal of the grammar is carried there. The ending that marks a plural noun, the ending that marks a finished action and the ending that marks a present verb with a third-person subject are, in the ordinary case, a single consonant fastened to the end of the word; only after a few particular sounds does one of them take a small vowel of its own and add a whole syllable. They are never stressed, but they are pronounced. English also allows two or more consonants to close a syllable together, with no vowel between them and none after the last of them. A word does not stop at the vowel unless the spelling says it does.
Speakers of languages whose syllables normally end in a vowel do one of two things here, and both are damaging. Some let the final consonant disappear altogether. Others keep it but add a small vowel after it, turning one syllable into two and making the word unrecognisable. Either way the listener stops receiving the grammatical endings, and three systems fail at the same moment: number, past time, and the present form used with a third-person subject. The error is tempting because those endings are unstressed and genuinely quiet in fluent speech, so they seem optional; and since the learner writes them correctly and knows the rule, they are sure they have said them. Record yourself and listen to the last sound of every word.