This is the 980th most frequent Chinese word.
只有
"Only" or "only if."
The word '只有' is used here to denote the condition required to achieve the outcome, emphasizing exclusivity.
只有努力学习，才能取得好成绩。
Only by studying hard can one achieve good results.
The word '只有' is applied to indicate limitation in a quantified sense, showing that he does not have more than one pen.
他只有一支笔。
He only has one pen.
The word '只有' emphasizes that among all other trees, this specific tree holds the exclusive characteristic of being the oldest.