Question: This was from my textbook and its confusing me:

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> 1. Tanya went to the country with a friend - Таня ездила в деревню с другом
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Why is it ездила and not ездили? Aren't Tanya AND a friend going, which would make it plural?

Another example from my textbook uses the plural past tense:

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> 2. Лена с Шариком пришли от ветеринара
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Can anyone explain why in the first case its the singular form and in the second case its in the plural form ?

Answer: 
I believe it's 

1. Because in the 1st sentence the predicate immediately follows the subject bringing it into the focus, therefore singular sounds more natural.
2. Because the idea is that it's Таня who went to the country, the presence of the friend is only secondary.

This word order would also fit an answer to the question **`С кем Таня ездила в деревню?`** in which case the predicate form is predetermined by the question.

On the other hand if the predicate followed both the subject and the complement, more natural (in my opinion) would be the plural form

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> Таня с другом ездили в деревню.
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which is kind of corroborated by your 2nd example.

This is a straightforward declarative sentence word order.

So the word order may depend on the idea the speaker/author is trying to convey, on their decision what part to emphasize.
In *sentence 1* only **Таня** is the *subject* while the words **с другом** just describe the details of *verb's action* (**went** with whom). 

In *sentence 2* **Лена с Шариком** is a *compound subject* (**Лена** is a *nominal subject in the Nominative case* **+ с Шариком** is a *nominal subject in the Instrumental case*) which can be referred to as **они** (plural), so the parts of the subject 'participate' in a joint action:

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> [**Составное подлежащее:**](https://www.calc.ru/Podlezhashcheye.html)
>  именное подлежащее в именительном падеже +
>  именное подлежащее в форме творительного падежа с предлогом «с»
>  (значение *совместности*):
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> *Девочка с собачкой вышли прогуляться.*
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