Final answer:
The sentence provided is incorrect and not complete. The subject 'we' should lead to a sentence stating 'We had an abundant harvest,' making it a complete sentence reflecting the success of the garden harvest.
Step-by-step explanation:
The sentence "Is we had an abundant harvest from our garden this year, we have plenty of squash and tomatoes." is not a complete sentence. The correct form would be: "We had an abundant harvest from our garden this year; we have plenty of squash and tomatoes." The subject of this complete sentence is we, and it conveys the idea that the speaker and others involved had a successful garden harvest, yielding a lot of squash and tomatoes.