sub

Compared meaning: an underwater military vessel (noun).

比较义项：潜水艇（名词）。

• submarine

  The standard, complete word for the vehicle; neutral in register and equally natural in formal, technical, and everyday contexts.

  这是表示该交通工具的标准、完整说法，语域中性，在正式、技术性和日常语境中都同样自然。

  The submarine remained submerged for nearly three months during the mission.

  在这次任务中，这艘潜水艇几乎潜伏了三个月都没有浮出水面。

• sub 本词

  An informal, clipped form of 'submarine', common in casual speech and military slang; sounds too breezy for formal or technical writing.

  这是 submarine 的非正式缩略形式，常见于口语和军事俚语中，用在正式或技术性写作里会显得过于随意。

  The old sub was so cramped that sailors could barely stand up straight.

  那艘旧潜艇非常拥挤，水手们几乎直不起腰来。

Both words mean the same underwater vessel, but the register differs. Submarine is the standard, complete word, natural in formal or everyday contexts. Sub is an informal, shortened form, common in casual speech and military slang. Sub's other meanings, like a sandwich, are not compared.

这两个词在“潜水艇”这一意思上很接近，submarine 是标准、正式的说法，sub 是它的非正式缩略形式，常见于口语和军事俚语。此处不比较 sub 的其他意思，例如“潜艇三明治”或“替补”。