いれ

わたしは コーヒーに さとう を いれ ます。

The correct answer is `いれ` (ire). This is the stem of the verb `いれる` (to put in/to add). It correctly combines with `ます` to form `いれます` (put in - polite present tense). The direct object `さとう` (sugar) is marked by `を`. So, `さとうをいれます` means "put in sugar." `さとう` (sugar) is the direct object and should precede `を`. `を` is the object particle and should follow `さとう`. `ます` is the polite ending and should follow the verb stem `いれ`. *Grammar Tip:* The pattern for transitive verbs in polite form is [Object] + `を` + [Verb Stem] + `ます`. `に` indicates the destination.

正解は「いれ」。「いれる」語幹で「いれます」と動詞が完成。「さとう」は「を」の前。「を」は助詞。「ます」は語尾。 文法：[目的語]＋「を」＋[動詞語幹]＋「ます」。