Agreement of case
Agreement in grammar, or concord, is the phenomenon in grammar where words used in certain contexts must match, in inflection, with certain other words in terms of such qualities as number and gender. It is not as strong a feature of correct language in English as it is in many other languages. This article is about agreement in case.
Case is a more important feature of foreign languages than of English. Case - briefly - is changing the ends of words according to the jobs they are doing in the sentence. It hardly exists in English except for the personal pronouns (like he, which is the subjective, or nominative, case, as opposed to him which is the objective, or accusative, case) and relative pronouns ('wh-words'), who and whom - again, subjective and objective respectively). In English, these pronouns are the only place where the difference between objective and subjective cases still matters.
One problem for native speakers of English lies in using the subjective form of pronouns like I. Much colloquial English, particularly among young speakers, uses the objective
me in such sentences as 'Jane and me went out.' Here, because [Jane and the speaker] is the subject of the verb, it should be 'Jane and I went out.' (It can help to think about whether you would say the single " Me went out", which seems obviously wrong to native-speakers.) Even more so, 'She and I went out' is more correct than ' Her and me went out' - even though this can sound very stiff to native ears. But " Us went out" sounds very odd in most dialects, and formal English always has "We went out".
A further problem comes when people apply the rule about using the subjective form with verbs to using it with prepositions, where it is incorrect. For details, see Case - agreement of preposition with pronoun.