English Grammar — Error Spotting (Fixed Pattern: able to + verb) Identify the part that contains a grammatical error. If there is no error, choose 'All correct'. Full sentence: One should be able for differentiate between what is desirable and what is not.

Introduction / Context:
This question focuses on the fixed infinitival pattern after “able.” In English, “able” is followed by “to + base verb.” Using a different preposition breaks the construction and is unidiomatic.

Given Data / Assumptions:

• Sentence: “One should be able for differentiate between what is desirable and what is not.”
• Exactly one fragment is wrong.

Concept / Approach:

The correct pattern is “be able to + verb” (e.g., able to swim, able to speak). “Able for” is used only in limited dialectal/older senses (e.g., “fit for”), not with verbs. Hence fragment A is erroneous; it should read “able to.”

Step-by-Step Solution:

1) Locate the adjective complement pattern after “able.”2) Replace “for” with “to” to form the infinitive: “able to differentiate …”.3) Confirm remaining fragments are grammatical and idiomatic.4) Select A as the error.

Verification / Alternative check:

Recast: “One should be able to differentiate between what is desirable and what is not.” This is the textbook construction.

Why Other Options Are Wrong:

B–D: All parts read naturally once A is corrected.

Common Pitfalls:

Transferring prepositions from other languages (e.g., “capable of” → “able for”). Memorise: able to + V; capable of + V-ing.

Final Answer:

One should be able for