Before coming to Spain, I thought Spainit was very hot and hardly had any raining.

HNowever, when that I live in Spain, however, it's quite cold sometimes and there are definitely rainy days.

We try to avoid using "however" at the beginning of a sentence.

I added the word "definitely" to emphasize that are indeed rainy days in Spain since you hadn't thought so before. The word "indeed" can be swapped out for "definitely."

The other day, it snowed oin mainland Spain.

We usually use the particle "in" instead of "on" when referring to place names.

Before coming to Spain, I thought Spain was very hot and hardly had raining.

Alternatively, "and that it hardly rained".

However, when I lived in Spain, it' was quite cold sometimes and there ware rainy days.

Even though Spain probably still has the same weather, if you're talking about your experiences when you lived in Spain, you have to use the past tense.

Alternatively, if you want to use the present tense (for example to emphasize that "this is how the weather in Spain is", rather than "this is the weather that I experienced was like"), you could rewrite the sentence as "However, when I lived in Spain, I realized that it's quite cold sometimes and that there are rainy days" or alternatively, "However, living in Spain made me realize that it's quite cold sometimes and that there are rainy days"

The other day, it snowed oin mainland Spain.

Before coming to Spain, I thought Spain was very hot and hardly ever raininged.

However, whennow that I live in Spain, I know it's quite cold sometimes and there are rainy days.

The other day, it snowed oin mainland Spain.

You could also say "on the Spanish mainland".

Before coming to Spain, I thought Spain was very hot and that it hardly raininged.

However, whennow that I live in Spain, it's quite cold sometimes and there are rainy days.

The other day, it snowed oin mainland Spain.