flat

Compared meaning: smooth and without slopes or gaps (adjective).

比较义项：表面平坦、无起伏或与相邻面齐平（形容词）。

• level

  Level describes a surface with no tilt, perfectly horizontal, often checked with a tool or against gravity; typically used for floors, tables, and ground.

  level 强调表面没有倾斜，完全水平，常靠工具或重力来检验，多用于地板、桌子、地面等。

  Check that the washing machine is level, or it will vibrate loudly.

  检查一下洗衣机是否放平，不然它运转时会剧烈震动。

• flat 本词

  Flat describes a surface without bumps, curves, or slopes, evenly smooth overall, without implying it has been checked for horizontal tilt; used for land, roads, and surfaces generally.

  flat 强调表面没有凹凸、弯曲或坡度，整体平滑，不一定涉及是否水平的检验，多用于土地、道路及各种表面。

  The desert stretched out flat in every direction, without a single hill.

  沙漠向四面八方延伸得十分平坦，没有一座山丘。

• flush

  Flush describes one surface being exactly even with an adjacent surface, with no gap or step between them; it is always used in relation to another surface.

  flush 强调某一表面与相邻表面完全齐平、之间没有缝隙或高低差，总是相对另一个表面而言。

  Sand down the patch until it's flush with the rest of the wall.

  把这块补丁打磨到与墙面其余部分齐平为止。

All three describe a flat surface. Level means horizontal, with no tilt. Flat means smooth, without bumps or curves. Flush means one surface is exactly even with the surface beside it. Other senses of these words are not compared here.

这三个词都能形容“平坦”的表面，但侧重点不同：level 强调水平、无倾斜；flat 强调没有凹凸起伏；flush 强调与相邻表面对齐、齐平。这里不比较 level 的“程度、水平”义（如例句中的 advanced level）、flat 的“公寓”义（如例句中的 a small flat）以及 flush 的“脸红”义（如例句中的 begin to flush）。

Compared meaning: dull, lifeless, or lacking variation (adjective).

比较义项：单调、缺乏变化或活力（形容词）。

• flat 本词

  Describes a single voice, tone, remark or performance as lacking energy or feeling, sounding emotionless or dull in that moment, not necessarily repetitive.

  形容某一次的声音、语气或表现缺乏活力或感情，听起来平淡、没有生气，不一定是因为重复。

  She said 'thanks' in such a flat voice that he wondered if she was upset.

  她用一种很平淡的声音说“谢谢”，他不禁怀疑她是不是不高兴了。

• monotonous

  Describes a voice, sound, job or routine that stays the same without variation over time, becoming boring because it never changes.

  形容声音、工作或日常生活长时间一成不变，因为毫无变化而让人感到乏味。

  The teacher's monotonous voice droned on for an hour, and half the class fell asleep.

  老师用单调的声音讲了一个小时，班上一半的同学都睡着了。

Flat and monotonous both describe something dull, but differently. Flat means a voice, remark, or performance lacks feeling right now. Monotonous means a voice, sound, job, or routine stays the same over time, becoming boring. Flat's other sense, 'apartment', is not compared here.

这两个词在描述声音、语气或表现缺乏生气时意思相近，但侧重不同：flat 强调那一刻缺乏情感和活力，给人平淡、没劲的感觉；monotonous 强调因反复、不变而让人感到乏味。flat 的其他义项（如“公寓”“平的”）不在此比较范围内。