共（とも） is a noun in Japanese that is frequently used as a either a 接頭語（せっとうご） (prefix), or a 接尾後（せつびご） (suffix). In the case of
と共（とも）に, it is used in conjunction with the 格助詞（かくじょし） (case marking particle) と as an adverb.
と共（とも）に expresses that (A) will happen ‘together with (B)’, or ‘at the same time as (B)’.
と共（とも）に may be used after verbs,
い-Adjectives,
な-Adjectives or nouns. With
な-Adjectives, it will sometimes be preceded by
である.
-
風（かぜ）が強（つよ）くなると共（とも）に雨（あめ）が降（ふ）ってきます。
As the wind gets strong, it will start to rain.
あの先生（せんせい）は厳（きび）しいが、厳（きび）しいと共（とも）に優（やさ）しい。
That teacher is strict, but as strict as he is, he is also very kind.
僕（ぼく）が住（す）んでいる場所（ばしょ）は静（しず）かであると共（とも）に、空気（くうき）が綺麗（きれい）だ。
The place I live is very quiet, together with the air being clean.
私（わたし）と共（とも）に人生（じんせい）を歩（あゆ）んでくれませんか。
Would you please walk together with me forever. (歩（あゆ）む is a poetic way of saying ‘to walk’ that often means ‘to travel a passage’)
と共（とも）に is a structure that is used when two things are ‘existing’ in the same way. This means that it may be used to show mutual actions, or that it may be used to show mutual qualities (states of existence). Because of this nuance ‘to be mutually (A)’ is one of the closest literal translations of
と共（とも）に. (A) may be an action, an adjective, or even a noun!
-
年（とし）を取（と）ると共（とも）に、身体（からだ）がどんどん硬（かた）くなってきた。
As I get older, I am starting to get less flexible.
この池（いけ）は綺麗（きれい）であると共（とも）に、危険（きけん）である。
This lake is beautiful and dangerous at the same time.