As mentioned in the previous post about sentence stress, under normal sentence stress, content words are emphasized and structure words are not. However, there are cases when some structure words might be given emphasis in a sentence. This happens when the speaker wants to convey something important to the listener which he would not be able to do if he uses a sentence which is neutrally stressed. See the examples below:
KEI didn't say that he will accompany Rose tomorrow. (Perhaps it's not Kei but someone else who said that)
Kei DID'NT SAY that he will accompany Rose tomorrow. (Kei probably just gave a hint about it but did not actually say it out loud)
Kei didn't say that HE will accompany Rose tomorrow. (Kei may not be talking about himself. He might be referring to someone else who will accompany Rose.)
Kei didn't say that he will ACCOMPANY Rose tomorrow. (Kei might fetch Rose, not necessarily accompany her)
Kei didn't say that he will accompany ROSE tomorrow. (It might not be Rose but another girl Kei will accompany.)
Kei didn't say that he will accompany Rose TOMORROW. (Kei might accompany Rose on another day, not necessarily tomorrow.)
As you can see in the examples above, the exact same sentence might have various meanings depending on what word is given stress; that's how important sentence stress is.
"Who" is used as the subject of the verb, thus if "he", "she" feels right, you can use "who". Being the subject of the verb means it is the one doing the action, like in the example below. She is the one who stole someone's heart, thus "who" is used.
Ex. She is the one who stole my heart.
"Whom" is used as the object of the verb or preposition. "Her" here is the "whom" referred to. She didn't do anything. It was he who went out on a date and he who admires her.
Ex. He went out on a date with her, whom he admires very much.
You simply have to look for the verb and see who's doing the action, to know if you ought to use "who" or "whom".
Sentence stress is emphasizing certain word in a sentence in order to convey the meaning of the spoken sentence. It is that which gives rhythm/beat and meaning to the sentence.
Under normally/neutrally stressed sentences, the words given emphasis are the content words. We usually just glide over the structure words.
Content words-words that carry the meaning of the sentence; important words in the sentence (main verbs, nouns, adjectives, adverbs and negative auxiliaries are considered content words in a sentence)
Structure words-words that make the sentence grammatically correct but are not that necessary to make the sentence understandable (pronouns, prepositions, articles, auxiliary verbs and conjunctions are considered structure words)
In this SAMPLE SENTENCE, the WORDS in CAPITAL LETTERS are the CONTENT WORDS and those in LOWER CAPS are the STRUCTURE WORDS.
Sentences under normal stress can be understood clearly even without clearly hearing the structure words so long as the content words are understood.
Kinds of Sentences According to Use Quiz