diff --git a/apps/web/src/oj/classroom/components/ExplainModal.vue b/apps/web/src/oj/classroom/components/ExplainModal.vue
index 4e27a46..52a4805 100644
--- a/apps/web/src/oj/classroom/components/ExplainModal.vue
+++ b/apps/web/src/oj/classroom/components/ExplainModal.vue
@@ -15,11 +15,15 @@ import { useThemeVars } from "naive-ui"
 import SubmissionView from "oj/submission/components/SubmissionView.vue"
 import UserName from "shared/components/UserName.vue"
 import type { ClassBoardProblem } from "utils/types"
+import ProblemBrief from "./ProblemBrief.vue"
 
 /**
  * 课堂看板「讲错误」（设计稿 BdOpen）：点一个红格子或错法堆里的名字打开。老师在讲台上
  * 一个个讲这道题错着的人，所以顶上是这几个人，← → 换人；下面和独立的提交详情页同一块
  * （代码占满宽、为什么错在后），只是不出名字那一行、点「第几次」就地换。
+ *
+ * 「题目」按钮：演示错误时题目和代码要并排（用户要的），点了左边出题目，再点收起。
+ * 平时收着，代码照样占满宽。开没开记在这台电脑上，讲下一个人、下一道题还是那样
  */
 const props = defineProps<{
   problem: ClassBoardProblem | null
@@ -44,6 +48,7 @@ const vars = computed(() => ({
   "--hover": theme.value.hoverColor,
 }))
 const current = computed(() => props.people[index.value] ?? null)
+const showProblem = useStorage("oj_explain_show_problem", false)
 
 function step(delta: -1 | 1) {
   const next = index.value + delta
@@ -87,6 +92,14 @@ onKeyStroke(["ArrowLeft", "ArrowRight"], (event) => {
         </div>
         <span v-if="people.length > 1" class="keys"><kbd>←</kbd><kbd>→</kbd> 换人</span>
         <div class="spacer"></div>
+        <n-button
+          v-if="problem"
+          :type="showProblem ? 'primary' : 'default'"
+          :secondary="showProblem"
+          @click="showProblem = !showProblem"
+        >
+          题目
+        </n-button>
         <n-button
           v-if="current"
           tag="a"
@@ -99,7 +112,14 @@ onKeyStroke(["ArrowLeft", "ArrowRight"], (event) => {
           <template #icon><Icon icon="ph:x-bold" /></template>
         </n-button>
       </div>
-      <SubmissionView v-if="current" :id="current.submissionId" embedded />
+      <div class="body" :class="{ split: showProblem && problem }">
+        <aside v-if="showProblem && problem" class="statement">
+          <ProblemBrief :display-id="problem.problemDisplayId" />
+        </aside>
+        <div class="main">
+          <SubmissionView v-if="current" :id="current.submissionId" embedded />
+        </div>
+      </div>
     </div>
   </n-modal>
 </template>
@@ -197,4 +217,25 @@ kbd {
 .spacer {
   flex: 1 1 0;
 }
+
+.body.split {
+  display: grid;
+  grid-template-columns: minmax(300px, 2fr) minmax(0, 3fr);
+  gap: 20px;
+  align-items: start;
+}
+
+/* 题目自己滚：讲到代码后半段时题目还在旁边 */
+.statement {
+  position: sticky;
+  top: 0;
+  max-height: calc(100vh - 140px);
+  overflow: auto;
+  padding-right: 8px;
+  color: var(--text1);
+}
+
+.main {
+  min-width: 0;
+}
 </style>
diff --git a/apps/web/src/oj/classroom/components/ProblemBrief.vue b/apps/web/src/oj/classroom/components/ProblemBrief.vue
new file mode 100644
index 0000000..eaca0a6
--- /dev/null
+++ b/apps/web/src/oj/classroom/components/ProblemBrief.vue
@@ -0,0 +1,122 @@
+<script setup lang="ts">
+import { isSqlProblem } from "@oj2/contract"
+import { getProblem } from "oj/api"
+import Markdown from "shared/components/Markdown.vue"
+import { errorMessage } from "utils/api"
+import type { ProblemDetail } from "utils/types"
+
+/**
+ * 「讲错误」弹层左边的题目（用户要的：演示学生的错误时题目和代码要并排）。
+ * 只摆讲题用得上的：描述、输入、输出、例子 —— 题目页的状态标签、流程图、相似题都不要。
+ */
+const props = defineProps<{ displayId: string }>()
+
+const problem = ref<ProblemDetail | null>(null)
+const failed = ref("")
+
+// 同一道题 ← → 换人不重拉：弹层开着时一直是这道题
+const cache = new Map<string, ProblemDetail>()
+watch(
+  () => props.displayId,
+  async (id) => {
+    failed.value = ""
+    const hit = cache.get(id)
+    if (hit) {
+      problem.value = hit
+      return
+    }
+    problem.value = null
+    try {
+      const data = await getProblem(id, "")
+      cache.set(id, data)
+      if (props.displayId === id) problem.value = data
+    } catch (error) {
+      failed.value = errorMessage(error)
+    }
+  },
+  { immediate: true },
+)
+
+const isSQL = computed(() => !!problem.value && isSqlProblem(problem.value))
+</script>
+
+<template>
+  <div class="brief">
+    <n-text v-if="failed" type="error">{{ failed }}</n-text>
+    <n-spin v-else-if="!problem" size="small" />
+    <template v-else>
+      <section class="block">
+        <h3 class="title">描述</h3>
+        <Markdown html :content="problem.description" />
+      </section>
+      <template v-if="!isSQL">
+        <section v-if="problem.inputDescription" class="block">
+          <h3 class="title">输入</h3>
+          <Markdown html :content="problem.inputDescription" />
+        </section>
+        <section v-if="problem.outputDescription" class="block">
+          <h3 class="title">输出</h3>
+          <Markdown html :content="problem.outputDescription" />
+        </section>
+        <section v-if="problem.samples.length" class="block">
+          <h3 class="title">例子</h3>
+          <div v-for="(sample, i) in problem.samples" :key="i" class="sample">
+            <div class="io">
+              <span class="label">例子 {{ i + 1 }} 输入</span>
+              <pre>{{ sample.input }}</pre>
+            </div>
+            <div class="io">
+              <span class="label">输出</span>
+              <pre>{{ sample.output }}</pre>
+            </div>
+          </div>
+        </section>
+      </template>
+    </template>
+  </div>
+</template>
+
+<style scoped>
+.brief {
+  display: flex;
+  flex-direction: column;
+  gap: 14px;
+  font-size: 16px;
+  line-height: 1.75;
+}
+
+.title {
+  margin: 0 0 4px;
+  padding-left: 8px;
+  border-left: 3px solid var(--success);
+  font-size: 15px;
+}
+
+.sample {
+  display: grid;
+  grid-template-columns: minmax(0, 1fr) minmax(0, 1fr);
+  gap: 8px;
+  margin-bottom: 8px;
+}
+
+.io {
+  min-width: 0;
+  border: 1px solid var(--divider);
+  border-radius: 6px;
+  padding: 4px 10px 6px;
+}
+
+.label {
+  font-size: 12px;
+  color: var(--text3);
+}
+
+pre {
+  margin: 0;
+  font-family: Consolas, Monaco, monospace;
+  font-size: 14px;
+  line-height: 22px;
+  white-space: pre-wrap;
+  word-break: break-all;
+}
+</style>