1. I spend / 3 hours / browse online / clothes / I not buy anything.

Completed Sentence: I spent 3 hours browsing online for clothes, but I didn't buy anything.

Explanation: We need to use the past tense "spent" since the action is finished. "Browsing" is used as a gerund (verb acting as a noun) after the preposition "for". "Didn't" is the correct negative auxiliary verb for the past simple.

2. All the items / a supermarket / sell / fixed prices.

Completed Sentence: All the items sold in the supermarket are at fixed prices.

Explanation: "Sold" is the past participle of "sell" and acts as an adjective describing "items". "At fixed prices" is the correct prepositional phrase.

3. Nick / pride himself / be able / speak Spanish fluently.

Completed Sentence: Nick prides himself on being able to speak Spanish fluently.

Explanation: "Prides" is used in the present simple tense as it's a habitual action. "On being able" is the correct structure after the verb "pride oneself".

4. Shopping online / offer / advantages / than shopping offline.

Completed Sentence: Shopping online offers more advantages than shopping offline.

Explanation: We use the comparative form "more advantages than" to show the difference between online and offline shopping.

5. My mom / never / buy anything / that dollar store.

Completed Sentence: My mom never buys anything at that dollar store.

Explanation: "Never buys" is the present simple tense used for habitual actions. "At that dollar store" is the correct preposition to indicate location.