It may be confusing and hard to know when to use "of" and when to use "off" in your writing. Frankly speaking, there is actually little you can do to logically understand these. You have to make sure you memorise these for good, and simply learn by heart every expression that uses "of" and every expression that uses "off" in order to correctly use them in your own sentences.
So, without further adieu, let's get started with some short explanations so you can understand these two words with relevant examples that will illustrate how to correctly use them in your written and spoken English!
Of vs. Off
There isn't much to say about the differences between "of" and "off", at least not much that can actually be helpful to remember the contexts that require the use of the former or the latter. Even so, it's worth mentioning that "of" is always a preposition, never having any other grammatical function in a phrase, whereas "off" can be used as a preposition, adverb or adjective as well. Read more about each of these words below!
When do we use "of"?
As a preposition, "of" can be used with several different meanings, in multiple phrases. First of all, it is used to indicate belonging to someone or something. It can also be used after words that indicate the amount of something.
Thirdly, "of" is a preposition frequently used with dates, percents, numbers or ages. It can also be added after a noun that contains something, with exactly this meaning: containing. The same case is when "of" is used to refer to the material that something is made from.
“Of" is also a preposition that may connect certain adjectives or verbs with nouns. It can also be used to indicate causality, as synonym for "due to the reason" or "due to the cause"; or as part of word structures that indicate positions.
Example 3: It was the 21st of January. - "of" can appear in numbers, percents, ages or, here, in dates.
Example 7: She died of heart attack. - "of" can also show causality.
When do we use "off"?
As an adverb or preposition, "off" is mainly used when referring to something that is not touching, not on a surface, or not connected to something else. It can also be used with the significance of "away"; or when referring to an object not operating, or a person not on duty. It can also be used to indicate a location next to another, or near a building; or when referring to a price that has been deducted. With a secondary meaning, it can be used when referring to something far in time or distance.
When "off" is used as an adjective, it should never appear before a noun. As for the meanings it carries, "off" can appear as an adjective describing something that is not correct. When describing food, "off" is used for food or drink that is not fresh or good to consume. And, eventually, "off" can also be used as an adjective to describe someone that is not at work.
As you have already observed with the examples above, "of" and "off" are both complex words, with multiple, distinct meanings and expressions that contain them. The best way to use them correctly is, unfortunately, not to find a logical explanation for them (which doesn't really exist), but to simply use your memory to learn them by heart.