Hi, this looks very good to me. I'll have to be extra pedantic to find some errors. I'm no expert but I am a native speaker, so I'll give you a couple of pointers.

The thing that sticks out most to me is the long paragraph - this makes it a little more difficult to read. I would start a new paragraph after 'the second group', and 'the third group'.

A few other minor grammatical mistakes: ...'the development of a country is measured by the Human Development Index...' [addition of 'the']. Also 'that consists' [add 's'].

I would normally put commas around 'in general', or I would replace it with something else like 'normally' or 'tend to be', otherwise it sounds like 'these countries are in general countries' - what's a 'general country'? Sometimes a good use of thumb to use is to see whether the phrase can be put in brackets, in which case commas are probably needed i.e. 'these countries are (in general) countries...' - which would work fine.

'Some of developing countries' should be 'Some developing countries'.

Another point that I would mention is the use of 'is' and 'are'. You wrote 'The first group is called developed countries' - now this is technically correct (as I understand it) but there may be people who would write 'The first group are called developed countries'. Apparently there's a difference between American use and English use here and perhaps also between the use by pure grammatists and popular use. It might be helpful if someone could shed some more light on this.

An example I found was:
When was Manchester Utd founded? It was founded in 1878.
Where do Manchester Utd play? They play at Old Trafford.

So what you have written is perfectly correct, it's just that you may see alternative uses preferred.

A final note on the content: I thought that LDCs and MDCs had been scrapped for being politically incorrect and were replaced by 'LEDCs' and 'MEDCs' (Less ECONOMICALLY developed countries). I can't keep up with the changes in terminology!

Henry

Kinds of countries : developed, developing, and underdeveloped

The second group are the developing countries. These countries are generally those that <strike>in general countries which have</strike> do not have <strike>achieved</strike> a high degree /significant degree/ of industrialization, <strike>compared to their population,</strike> and in most cases, these countries have medium to low standards of living. Having said all that, it’s very hard to find/have /a single definitive definition for the term “developing countries”, since the level of development may vary widely from one country to another/ <strike>within</strike> among these/ such countries. Some of these developing countries have high average standards of living, yet it is wrong to <strike>and can be</strike> classify them <strike>neither</strike> as a developed country, nor a developing one.. So, they are  referred to as “newly industrialized countries”. Ukraine, Iran, and Chile are examples of developing countries; and China, Turkey, India, and Malaysia are examples of these so-called newly industrialized countries. This is according to a list published by/drawn up by the International Monetary Fund.

The third group of countries are the underdeveloped countries. The indicators of their low status <strike>Symptoms of which</strike> include the lack of <strike>access to</strike> of job opportunities in these countries, their poor health care system, a poor access to drinkable water, a lack of food, and their inabilities to provide good education and housing for its people. Examples of such countries are the many poor African countries that we know of,  where people have to live in deprived cities.

When you use "like" or "such as", you do not have to use etc., because those words are already indicating that you are going to name a few, not all of them.

"Such as " is inclusive than "like" . I like a girl like you ( not you but like you) I like a girl such as you ( you and any girl like you)

Thus and hence mean therefore . If unsure, just use therefore.