Read each sentence then write whether the sentence is simple or not simple

My dog Buddy fetches the paper in the morning
Simple

The subway which runs under the city is a fast way to travel
Not simple

Cici sings in the school choir
simple

Jen talks on the phone and does her homework at the same time
Not simple

When the train arrived vika boarded it quickly
simple?

The first 3 are correct.

Your first two are right. The second two are wrong.

The sentence about Jen has only one subject -- Jen.

The sentence about the train has two subjects and verbs.
train arrives
vika boarded

Correction -- I can't count. Of course, Writeacher is correct -- the first 3 are correct.

The one about Jen is Simple and the one with Vika is not Simple?

It would help if the sentences were numbered!!

The sentence "When the train arrived, Vika boarded it quickly" is not simple. To determine if a sentence is simple or not, we can look at the structure and complexity of the sentence.

In this case, the sentence has a dependent clause ("When the train arrived") followed by an independent clause ("Vika boarded it quickly").
A simple sentence typically consists of only one independent clause, without any dependent clauses. So, the sentence "When the train arrived, Vika boarded it quickly" is not simple.