Question
Why do we use a female form ("lach") in Modim?
Answer
"Modim anachnu lach" is used in Divrei ha-Yamim I 29:13. In Modern Hebrew "lecha" is masculine and "lach" is feminine, but it Biblical Hebrew "lach" can be masculine as well as feminine. The first example is the "lach" addressed to Adam in Gen.3:18.