The difference of は and が

Hi, how are you doing? In the UK, these few days are quite hot in temperature, but now it's get cooler now. I am so used to air conditioning that cannot help searching remote controller.

By the way, は is not pronounced as 'ha', general way of は. Instead, it is called 'wa'. This is because of histrical usage of subject particle は. こんにちは, Japanese 'Hello', is also contains same specific. The last は is called 'wa'.

これはペンです。

means, 'This is a pen.'

In this example は works totally the same as 'is'.

In Japanese sentences, です often comes at the end of sentences to add meanings of politeness. So when you translate 'is' into Japanese words, it can be sets of 'は' and 'です'.

And, why is it so important to know subjective particle in Japanese? That's because in Japanese sentences subject often be eliminated.

本をとって下さい

means, 'Would you take me the book'. However, subject 'you' is' completely gone. Subjects can be meanless because it's thought to be obvious. When there is particles, は　が, however, they must indicate existence of subject.

Secondly, let's see が.

が is also used after subjective nouns.

これがペンです。

also means, ' This is a pen.' As you see the translation, two sentences has no difference at all. However, there is. For example

彼はあの会社に勤めています。　He works for the company.

sounds natural. It shows his belongings and personality. On the other hand,

彼があの会社に勤めています。

is possible, but not usually It sounds he is working for the company and the other is... It compare with other persons and 'he' is treated more systematically.

は focus on the person him/ herself and lead to additional comversation about the person. On the contrary, が shows information about the situation or person and can lead to additional information.

On English translation, the difference is none, but there is differences that what conversations comes after.