As usual, Lucid is spot on. It is generally not a good idea to use a single word keyword. There might be some exceptions where it makes sense to use a single word keyword, but those rare cases are few and far between, and usually involves a word that typically is only used to describe the product or service you are marketing.
Try to remember a keyword is just a tool used to target search terms. It is the search terms that you are targeting and your ad groups need to focus on a singular intent to perform well. A single word keyword is usually too vague or ambiguous to accurately target as narrowly as needed for your ad message to be highly effective.
In those rare cases where you do need to target a single word keyword, it is usually best to use only exact match keywords. If you are trying to get your impressions on phrases then you should be targeting those phrases explicitly, and a single word keyword is not explicitly targeting anything except that single word search term. So, my advice is to only use exact match for any single word keywords you need to target.
Don’t try to get away with using single-word keywords for broad or phrase match keywords, that’s just being lazy, and it is going to cost you a lot in performance. Do the upfront work of setting up proper targeting by explicitly targeting search terms and your wallet will thank you.