English grammar error-spotting (list-order adverb: ‘‘first’’ not ‘‘firstly’’ as sentence adverb): Read, detect the erroneous segment, and suggest the fix; consider idiomatic sequencing adverbs: ‘‘Firstly you should / think over the meaning of the words / and then use them. / No error.’

Given data

• A: ‘‘Firstly you should’’

Concept / Approach
As a sentence-opening sequencing adverb in formal style, first is preferred to ‘‘firstly’’ when only two steps follow (first … then …). ‘‘Firstly’’ is not wrong in all contexts, but standard test usage expects ‘‘first’’.

Step-by-step correction reasoning
Replace ‘‘Firstly’’ → ‘‘First,’’.

Corrected version
‘‘First, you should think over the meaning of the words and then use them.’’

Common pitfalls

• Starting steps with ‘‘Firstly’’ in two-item sequences (use ‘‘first … then …’’).

Final Answer
Firstly you should