復習をおろそかにしていては上達することはない。
- If you neglect review you will never improve.

復習をおろそかにしていては上達することはない。 meaning: If you neglect review, you will never improve.

⚫︎ The grammar pattern ～ては combines the te-form of a verb with the conditional particle は to express a negative conditional meaning equivalent to もし～たら or ～たら、悪い結果になる. ⚫︎ It indicates that if a certain negative state or action continues, it will inevitably lead to an undesirable outcome. ⚫︎ In this sentence, おろそかにしていては means もしおろそかにし続けているならば (if one continues to neglect). ⚫︎ This is a very common N1 pattern used to state warnings, rules, or logical consequences.

⚫︎ The phrase おろそかにする is a set expression derived from the adjectival noun おろそかだ (neglectful, careless) combined with the verb する. ⚫︎ It means to treat something lightly, neglect it, or fail to give it proper attention. ⚫︎ In this sentence, it modifies how one approaches 復習 (reviewing studies). ⚫︎ The noun 復習 is marked by the object particle を, forming 復習をおろそかにする (to neglect review).

⚫︎ The phrase ～ことはない is an N1 grammar pattern used to express strong negation, meaning 絶対に～ない (never do / there is no such thing as). ⚫︎ When attached to the dictionary form of a verb, it denies the possibility of that action or state occurring under the given condition. ⚫︎ In this sentence, it is attached to the verb phrase 上達する (to improve), creating 上達することはない (there is no chance of improving). ⚫︎ It provides a definitive and objective conclusion regarding the outcome of the action.

⚫︎ The verb 上達する is a Class 3 (suru-verb) intransitive verb meaning スキルや能力が向上する (to improve in skill or proficiency). ⚫︎ It conjugates regularly as a suru-verb, with its dictionary form being 上達する and its nominalized form using こと being 上達すること. ⚫︎ This verb frequently appears in contexts related to language learning, music, sports, and other trainable skills. ⚫︎ It acts as the core predicate inside the main clause of the sentence.

⚫︎ The particle を is the direct object marker, which attaches to the noun 復習 to show that it receives the action of the verb phrase おろそかにしている. ⚫︎ Although explicit conjunction particles are omitted between the clauses, the structure relies on the sequential relationship established by ～ては. ⚫︎ Understanding how particles connect nouns to actions is fundamental for parsing advanced Japanese sentences. ⚫︎ The interplay between the subordinate condition 復習をおろそかにしていては and the main clause 上達することはない creates a cohesive complex sentence.