בעלות על … (ל… לקבוע)

Name the person who has the final say

• A group asks who should select the gift. The decision is assigned to the child.
• to buy买mǎiwhat什么shén megift礼物lǐ wù，by由yóuchild孩子hái zioneself自己zì jǐdecide做主zuò zhǔ。 — The child is given authority over this particular choice.

Locate the pattern in real examples

• by由yóu … decide做主zuò zhǔ (for … to decide): this这zhè(measure word)件jiànmain content事由shì yóuyou你nǐdecide做主zuò zhǔ。; home家里jiā lǐ(modifier / structural particle)的demajor event大事dà shìby由yóufather and mother父母fù mǔdecide做主zuò zhǔ。
• Read the whole clause before choosing a translation. The examples show the pattern in use; the context below determines what the speaker claims.

Follow the speaker

this这zhè(measure word)件jiànmatter事shìby由yóuyou你nǐdecide做主zuò zhǔ。

• In the opening situation: to buy买mǎiwhat什么shén megift礼物lǐ wù，by由yóuchild孩子hái zioneself自己zì jǐdecide做主zuò zhǔ。
• The child is given authority over this particular choice.

Check the limit of the explanation

• This does not mean the child controls the entire event.
• State the area first when needed; decide做主zuò zhǔ assigns decision authority, unlike to help帮忙bāng máng, which only offers assistance.

Repair the form before interpreting it

• The form to buy买mǎiwhat什么shén megift礼物lǐ wù，by由yóudecide做主zuò zhǔchild孩子hái zi needs repair here. Write to buy买mǎiwhat什么shén megift礼物lǐ wù，by由yóuchild孩子hái zidecide做主zuò zhǔ instead; keep the word or frame in its demonstrated order.
• Form and meaning are separate checks. After the repair, explain the sentence in its situation rather than treating a familiar translation as a universal rule.

Keep the distinction

Read the full reference: HSK 6 grammar — Sentence patterns.