Current evaluation methods for large language models are coarse-grained and decoupled from generation, producing generic explanations that fail to provide actionable feedback for model improvement. We propose CriticGen, a fine-grained, generation-aware evaluation framework that turns evaluation into actionable control for answer improvement. CriticGen first generates sample-specific evaluation dimensions and scoring criteria, then uses the induced rubric to jointly produce a score, a reason, an executable refinement suggestion, and a refined answer. Experiments show that fine-grained evaluation should be both instance-specific and actionable. CriticGen improves human-rated rubric relevance/coverage from 3.33/4.03 to 3.97/4.24. On the score-consensus-filtered RefineData test set, the best CriticGen variant reaches 0.9556/0.9560 Pearson/Spearman agreement with the reference scores and improves reason/suggestion F1 to 0.7554/0.7900. Independent validation provides additional support: on 180 human-scored instances, CriticGen-Qwen2.5-7B obtains higher observed score correlations than GPT-5, with 0.8868/0.8969 Pearson/Spearman versus 0.8542/0.8615. Crucially, its feedback translates into substantial answer improvement, raising accuracy from 45.36% to 91.26% on 5,000 examples with reliable gold answers, with 47.30% of answers corrected from Wrong→Right and only 1.40% degraded from Right→Wrong.