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1. The delivery person was swell and he arrived right on time, but the restaurant food was sub par and has definitely worsened since the last time I ordered.

2. The delivery person was swell and he arrived right on time, but the restaurant food was sub par and had definitely worsened since the last time I ordered.

3. The delivery person was swell and he arrived right on time, but the restaurant food was sub par and has definitely worsened since the last time I had ordered.

Which one is grammatically correct? This was a food order from a few days ago.

I think the first one is the right one, but are we allowed to mix past and present perfect together?
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The use of 'swell' and 'subpar' immediately marks these utterances as casual, as does the linear composition, so I don't see that any of these can be called 'wrong'. Tenses of almost any sort can potentially appear in a single sentence; it depends on the relative times referred to.
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2. The delivery person was swell and he arrived right on time, but the restaurant food was sub par and had definitely worsened since the last time I ordered.

The past perfect is used to refer to a time that is earlier relative to another past time.

The time the pizza delivery man arrived is in the past. The time when the food was relatively good (the previous time you ordered) is an earlier time than that.

For example, let's say the last time you ordered pizza was three months ago (when it was good), and the delivery man arrived yesterday. In the previous three months (prior to yesterday), the quality of the food had deteriorated.
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Mister MicawberThe use of 'swell' and 'subpar' immediately marks these utterances as casual, as does the linear composition, so I don't see that any of these can be called 'wrong'. Tenses of almost any sort can potentially appear in a single sentence; it depends on the relative times referred to.
What do you mean by linear composition? And how would I make it not casual?
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By 'linear composition', I mean just writing a series of clauses attached to the end-to-end without really taking enough time to edit for style, subordination, redundancy, etc: the 'and-and-but-so' approach.

The delivery person was pleasant and prompt, but the food was even worse than in my previous order.