So I saw this sentence:
きのう初めておすしをたべました。 And wondered if the は is just being left out. Does the sentence change any if は is added in? Like so:
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I think this は is a "topical は" or "contrastive は". 昨日初めてお寿司を食べました sounds to me like you're just reporting that you ate sushi for the first time the day before, and 昨日"は"初めてお寿司を食べました sounds to me like "Speaking of yesterday, I ate sushi for the first time," (probably someone's just asked what you did yesterday, like 昨日、どこに行ったの? or 昨日、京都で何したの？etc.), or "(The day before yesterday I went to eat sukiyaki, and) as for yesterday, I ate sushi for the first time".
I'll venture to say that adding the は simply emphasizes yesterday. Without it it just feels like the fact that it was yesterday is a little less important.
There is no grammatical need for は in that sentence.
は is used for topic/comment constructions and this sentence isn't making a comment on the topic of "yesterday" at all.