在「Below, beneath, under, underneath (preps.)」一文中，筆者已就這幾個介系詞在指位置時它們意思的細微差異，做了詳細的解說。本文將就 below 和 under 與測量單位和數字連用來表示「低於；少於；小於」(less than) 的意思時常見的錯誤，做一說明。
年齡
在指年齡時要用 under，不用 below。然而，有些字典亦使用 below，讓人感到困惑，殊不知「未成年」的英文是 underage 而不是 belowage (沒這個字)。例如：
- We have three children under the age of six. (我們有三個不到六歲的孩子) (正)
- The nursery is open for children under 4. (這家托兒所收 4歲以下的兒童) (正)
測量單位
在指時間和重量時要用 under，不用 below。例如：
- The sports car can get from standstill up to 100 kilometers per hour in under five seconds. (這部跑車從靜止加速到時速 100 公里還不到五秒) (正)
The sports car can get from standstill up to 100 kilometers per hour in below six seconds. (誤) - A visa is not required for a stay of under six months. (停留六個月以內不需要簽證) (正)
A visa is not required for a stay of below six months. (誤) - This type of schoolbag was just under two kilos, so they could not hinder development in schoolchildren. (這款書包重量不到兩公斤，所以它們不會妨礙學童的發育) (正)
This type of schoolbag was just below two kilos, so they could not hinder development in schoolchildren. (誤) - The newborn baby weighed under three kilos. (這個新生兒重量不到三公斤) (正)
The newborn baby weighed below three kilos. (誤)
在指高度時要用 below，不用 under。例如：
- The roof of my new house is just below the height of the library. (我新房子的屋頂比圖書館的高度稍微低一點) (正)
The roof of my new house is just under the height of the library. (誤)
在指溫度時可用 below 或 under，但 below 是主流。例如：
- Last night it was two degrees below (zero) in Taipei. (昨晚台北只有零下二度) (正)
Last night it was two degrees under (zero) in Taipei. (誤) - The water must be kept below ten degrees Celsius. (這水必須保持在攝氏十度以下) － 這句亦可使用 under，但並不建議這樣做。
數字、數量、金額或水平通常與 below 連用；部分情況使用 under 也可以，但並不多見。例如：
- If you get below 50%, you fail the exam. (如果你考不到一半的分數，那麼就不及格)
- All items are/cost below ten NT dollars. (所有物品的價格均低於台幣十元)
- Jack’s test scores have been well below average. (傑克的考試分數遠低於平均水平)
- The company’s profits in 2016 were below what they had expected. (這家公司 2016 年的獲利比他們預期的來得低)