Do we speak and write differently in Mandarin/Chinese? Are there informal Mandarin phrases which we don't use in written Chinese?
To some extent yes, we speak and write differently in Mandarin/Chinese. Sometimes we speak certain phrases in Mandarin but we don't write that way. For instance when we want to say someone is free and we use the phrases "貓着" in spoken Chinese but we don't tend to use this phrase in our writing.
Are there singular or plural forms in written Chinese?
Unlike English, some of the words do not have plural forms in written Chinese. For instance, we use the article "a" to indicate there is only one horse (a horse) while we add "s" to indicate there are more than one horses. However, no matter how many horses we have, we simply use the word “馬“ to indicate horse/horses.
Jenn borrowed a bag from Lisa and she promised to return the bag with a keychain to her two months later at her request. Explain why the keychain can be treated as "interest" to both Jenn and Lisa.
To Jenn, the keychain was the price or cost she paid for the earlier availability of the bag whereas to Lisa, the keychain was the premium or compensation she received for deferring consumption.