言葉 means “words”.
は is the particle that indicates the topic of the sentence.
刃物 means “bladed objects”, “cutting tools” or “knives”.
ん is the broken version of the particle の, and when it’s used with the auxiliary verb だ, it expresses the speaker’s strong declaration based on a certain reason or background. It’s often used when explaining something to convince the listener.
It can be attached to a verb or i-adjective such as;
- 僕は行く (Verb) んだ！ meaning “I’m going!”
- ここのラーメンはおいしい (I-adjective) んだ meaning “the ramen here is delicious”
But when it’s attached to a noun or na-adjective you need to add な before んだ such as;
- 犯人は俺 (noun) なんだ meaning “the culprit is me”.
- この仕事は大変 (na-adjective) なんだ meaning “this job is hard”.
刃物 is a noun so な is added before んだ.
So 言葉は刃物なんだ directly means “words are knives” in an convincing way, but since it’s a metaphor we translate it as “words are like knives”.
There are two clauses in this sentence. 使い方を間違えると is the subordinate clause and 厄介な凶器になる is the main clause.
The stem of a verb masu-form followed by 方 means “how to do something” or “the way of doing something”. 使い is the stem of the verb 使います meaning “to use” so 使い方 means “how to use” or “the way of using”.
“Something を間違える” means “to get something wrong”.
And “The conjunction particle と following a verb dictionary form” means “if one does something”.
So the subordinate clause 使い方を間違えると directly means “if you get the way of using wrong” so it can be translated as “if you use them the wrong way”.
厄介な is a na-adjective meaning “troublesome” which is used for something causing a lot of problems. And it modifies the following word 凶器 meaning “murder weapons”. So 厄介な凶器 can be translated as “dangerous weapons”.
And “something になる” means “to become or turn into something”.
So the main clause 厄介な凶器になる means “they will turn into dangerous weapons”.