That was all he could tell them, but it was a clue, the first they had had in thirty years, and when they told the story of the lost child to their guest he was convinced that it was their son he had seen—there could be no other explanation of the extraordinary resemblance between the two young men.（A Traveller in Little Things by W. H. Hudson）注：he = their guest
英文を前から読んでいくために、語句のかたまりを作ります。こんな具合に課題文をスラッシュ（斜め線）で区切ります（スラッシュリーディング）。語句のかたまりを意識すると、リスニングで英語の意味をつかむのが楽になります。
That was all he could tell them／（接続詞の前）but it was a clue,／（コンマの後）the first they had had in thirty years,／（接続詞の前）and when they told the story of the lost child to their guest／（主語が文頭にないときの主語の前）he was convinced／（接続詞の前）that it was their son he had seen／（ダッシュの前）—there could be no other explanation／（前置詞の前）of the extraordinary resemblance between the two young men.
語句のかたまりを前から訳していきます。
That was all he could tell them
彼（客）が彼らに伝えられたのは、それがすべてだった
but it was a clue,
しかし、それは手がかりだった
the first they had had in thirty years,
30年間で彼らが得た初めての手がかり
and when they told the story of the lost child to their guest
そして、彼らが行方不明になった子どものことを客に話したとき
he was convinced
客は確信した
that it was their son he had seen
彼が見たのは彼らの息子だったと
—there could be no other explanation
そうでなければ説明できなかった
of the extraordinary resemblance between the two young men.
２人の若者が驚くほど似ていたことが
That was all he could tell them, but it was a clue, the first they had had in thirty years, and when they told the story of the lost child to their guest he was convinced that it was their son he had seen—there could be no other explanation of the extraordinary resemblance between the two young men.
彼（客）が彼らに伝えられたのは、それですべてだったが、それは手がかりだった。30年間で彼らが得た初めての手がかりだ。そして、彼らが行方不明になった子どものことを客に話したとき、客は自分が見たのは彼らの息子だったと確信した。そうでなければ、その２人の若者が驚くほど似ていることが説明できなかった。
That was all he could tell them, but it was a clue, the first they had had in thirty years, and when they told the story of the lost child to their guest he was convinced that it was their son he had seen—there could be no other explanation of the extraordinary resemblance between the two young men.
That was all he could tell them→ he の前の関係代名詞が省略されています。補うとすると、先行詞が all なので which ではなく that です。
the first they had had in thirty years→ この部分は直前の a clue を説明するための同格表現です。they の前の関係代名詞が省略されており、補うなら、先行詞が序数なので which ではなく that です。
it was their son he had seen→ 強調構文です。it is〜that―の that が省略された形です。→ it was their son that he had seen
動画の後半部分でリスニング用音声が流れます。
