He could have bought something to eat, couldn't he?

I think it’s time. We must leave now, mustn’t we?

Your son is collecting stamps, isn't he?

The children were yelling in the classroom, weren’t they?

You father can help you, can’t he?

I am doing it right, aren’t I?

They often watch TV till late in the evening, don't they?

Grammatically, "also" can stay in different places in a sentence and be correct, but... the place of "also" is important regarding the meaning and the emphasis of the sentence.

The purpose of also is to join two ideas.
Let's say  the first idea is that "I think something"
and the second one is "what I think":
Look at the different places of also and try to see the meaning;

Also, I think that my father should  buy a new car.
I also think that my father should buy a new car.
I think that my father also should buy a new car.
I think that my father should also buy a new car.
I think my father should buy a new car also.
The position of "also" changes the emphasis and the meaning.
What is "also" actually?
"Also" is an adverb, that's why its best position is near the verb, but if you want to focus on other part of the sentence, you can change the position.
"Also" highlights the word near it.

Well, this is about emphasis.

When it's not about the focus:

"Also" comes in affirmative sentences to add another one like the first:

Mother loves Al Pacino's films.

Dad  also loves him. I love sport. I also love a good meal.

"1. Also" comes after "to be."

I am also your friend.

2. but  before all other verbs.

I also run in the morning.

"also" stays after have or be and before the participle or verb+ing.

I have also finished the project.

I am also working a lot.

I can also play football.

Subject and Verb Agreement

Plural or singular?

The subject expresses how many people? One person or ...

Mother and father have gone to the market.

The cat and the dog do not love each other.

My teacher and friend is a wonderful person.

The team leader and the deputy were against the new project.

Each person has rights.

Each child brings huge value to the family.

Neither Jane  nor John has passed the exam.

 Either Mom  or Dad has called you. 

 Neither Grandpa nor Grandma were here yesterday.

Neither he nor I am guilty.

 Neither you nor he is a good sportman.

Determiners: neither, either, most, much, many...

Just a little about neither of,  neither,  either,  many, much or a lot of...

Neither of my parents can use a computer.   (verb affirmative, of + noun in plural)

 Neither of them goes to work.  (verb affirmative, of + pronoun in plural)

Neither test is perfect.  (verb affirmative, noun in singular without "of")

 ‘I can’t cook.’ -> 

‘Neither can I.’   (verb affirmative)

 ‘My mother can’t ride the bike.’  ->

‘I can’t  either.’   (verb negative + either)

 I don’t go out too much these days. (much -> uncountable)

 I haven’t got much time.  (much -> uncountable)

 My mother has got a lot of outfits but she doesn’t wear many accessories..  

 You haven’t slept much. (much -> uncountable)

 Most ham is made from pork in this restaurant. 

We use TO when we talk about a destination, what time it is, a distance - final point, when we compare something, when we give or explain something to someone and when we express a reason of an action, a purpose given with a verb.
I am going to Berlin
It’s ten minutes to twelve
There are 2 km from the center of the town to the stadion.
I always prefer meat to cheese.
I offered some beautiful flowers to my teacher.
I called to hear your voice.

We use FOR when we express some benefits, some period of time, a scheduale, when we agree with something, when we do something for somebody because we want to help, when we give a reason using a noun and some function using a verb with -ing form. 
The treatment is good for your health.
I have worked at this project for 3 months.
We have an appointment for April the 3rd.
I am for not against this candidate.
Could you write this email for me? I have no time and I should send it today. (this means "instead of me).
Let’s go out for a pizza.
A pen is a tool we use for hand writing.
To/for

I took the pen to write.

I took the pen for the test.