"Found" is the main verb of this structure. Before the main verb, we need a subject. Here "with his friend" is not a subject, so this is a prepositional phrase. "his friend" is the object of the preposition "with". An object couldn't consider as a subject of the sentence.

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1. Practice interleaving:

Interleaving is the idea of mixing or alternating skills or concepts that you want to memorize. For example, spend some time memorizing vocabulary words for your science class and then immediately switch to studying historical dates and names for your history class. Follow that up with practicing a few math problems, and then jump back to the science definitions. This method may seem confusing at first, but yields better results in the end than simply spending long periods of time on the same concept.

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1. Exercise:

Studies show that exercise can improve our memory and learning capabilities because it helps create neurons in areas that relate to memory. Cardio and resistance training (weights) both have powerful effects, so do what works best for you.

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? A friend of ours/ us??????‍♀

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This is called a “Double genitive”, (genitive possessive), because the “of” in the expression already shows possession, but we also use the possessive “ours”.
This is common across all pronouns and nouns.

e.g:

✅ She is a friend of mine.
❎She is a friend of me.

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