IOn athe way to the seed shop

When ithe sun was bright outside, my girlfriend and I were walking toward the shop wherethat was selling various seeds. / When it was sunny, my girlfriend and I were walking toward the shop that was selling various seeds.

Your sentence is correct, but these phrases sound more natural. :)

Little did I know that I was about to experience the hardest time fighting with mosquitos under the burstning sun.

I decided to ring my sister and her husband and ask when they will change theirget a new car.

They told me once that they had a plan to change theirget a new car for their baby.

Their baby will born in a few months later and they need thea car seat, but their car is old fashioned, so they can’t attach the newer version of the car seat to their car.

So they have wanted to change their car to a newer and bigger one.

My sister had helped my mom to file her income tax return as I called her.

I also had suffered from filing the income tax return which wasn’t just complicated to write but has also been changed a bit every years.

After catching up (on recent livesevents), we talked about the car.

Their car even had more things to check or, change or refill, such as a mission oil, coolant and blah blah blah.

I missed one thing: that their car was also got the agesold.

Despite the condition of their car, I was still havexperiencing a strong desire to get their car.

Because my car was made in 2004 and it’s condition was worse than their's.

It's = It is

So, it should not be used here. :)

The car mechanic once said to me that the coolant was totally rotten, meansing the component was rusty so replacing the coolant was useless because it would be contaminated again and again.

My girlfriend and I had looked around the used car market but actually I couldn’t believetrust them.

I bought my car at the used car market 5 years ago and as you heard, it’s condition was bad.

My sister’s husband said handing over their car would be difficult this year because they got thehad a housing problem to solve first.

We decided to keep our car for one more year.

In this way we might keep our car for long yearsmany more years. / In this way we might keep our car for a longer time.

Actually, we love our car.

When the sun was bright, my girlfriend and I were walking toward the shop where they were selling various seeds.

Meanwhile, our car was at the garage to change the engine oil and to refill the refrigerant of the air conditioner.

I decided to ring my sister and her husband andto ask when they will change their car.

They told me once that they had a plan to change their car for their baby.

Their baby will be born a few months later and they need the car seat, but their car is old fashioned, so they can’t attach the newer version of the car seat to their car.

To be born - 'be' must be put before born

So they have wanted to change their car to a newer and bigger one.

My sister had helped my mom to file her income tax return aswhen I called her.

I had also had suffered from filing the income tax return, which wasn’t just complicated to write but has been changed a bit every years.

After catching up about our recent lives, we talked about the car.

When I said my car was at the garage for repair, she said their car was at the garage, too.

Their car even had more things to check or, change or refill, s. Such as a mission oil, coolant and blah blah blah.

She told me that her husband told her that they might not be able to give their car to us.

I missed one thing that their car also got the ages.

This is a bit confusing. I am not sure what you mean. If you want to say the car is old, I suggest writing:
"I missed one thing about their car, it was also old."

Despite the condition of their car, I was still having a strong desire to get their car.

Because my car was made in 2004 and it’s condition was worse than theirs.

The car mechanic once said to me that the coolant was totally rotten,. This means the component was rusty so replacing the coolant was useless because it would be contaminated again and again.

My girlfriend and I had looked around the used car market but actually I couldn’t believe them.

The last part of your sentence "but actually I couldn't believe them" is stylistically confusing, but grammatically makes sense.

My sister’s husband said handing over their car would be difficult this year because they gothave their housing problem to solve first.

In this way we might keep our car for longmany years.

Before I knew it, we had arrived at the seed shop.