有關這句英文怎麼拆解文法，看不懂?

(Sniffer) then scampered along, stopping now and again when he smelled something good.
然後（牠）沿路蹦蹦跳跳，當牠聞到好東西的時候，偶爾停下來。

句子中加了個逗號(comma), 意思就清楚了。
"stopping now and again ..." 是一個分詞片語 (participle phrase) 描述句子主語 "Sniffer" 所做的動作。 (文法上，是分詞構句 ~ participle construction after verb of movement "scampered", describing the action done by the same subject of the main clause during the movement)
"now and again" 是一個 set phrase, 意思是 occasionally (有時，偶爾)

**** more example for participle construction
If the subject is different from the one of the main clause, preposition "with" can be used in front of the phrase.
e.g. (Sniffer) then scampered along, with his tail wagging when he smelled something good.

超級感謝你，很清楚！