There are a lot of people. The moment you put a before lot you dismiss lots. Also, a failure I used to do was "There is a lot of" or "There are a lot of" In that case, you have to figure out what can be counted and what not. If it can be counted than there are (like issues, problems, people) But if it is something uncountable than there is a lot of (water, fire, etc.) To make it sound more smooth use "plenty of" instead of a lot or lots of.

When I learned Turkish and English, I had the same problem. I would listen to news and people talk and would not miss a single beat of what they were saying. So a friend of mine suggested me to watch a 20 min sitcom in my language and afterward the translated version. You don't have to write something down. It just takes some time cause you have to watch episodes twice :D I did this with series like Big Bang Theorie, Dexter, and Dr. House and it helped.

The second part: Dieser, Diese, Dieses is way more tricky first of all you have to know that these are demonstrative pronouns which means they each also belong to one gender. So for example, Der Baum, Dieser Baum Die Katze, Diese Kate Das Spiel, Dieses Spiel I would suggest you memorize the endings "r," e," s." So Das and Dieses have both "s" at ending. Die and Diese have both "e" at the ending and Der, and Dieser also have both "r" at ending. So by doing this, you have the right one at the right place. So to use that in sentences. Kannst du mir bitte das Brot geben? Can you give please give me the bread? Gib mir bitte dieses Brot! Give me please that bread! So the first sentence was not a specific bread, I asked for the bread, and you would hand me whatever one you find handy over.But the second question was a specific bread. So If you say Gib mir bitte das Brot and point on a specific one like you would see in bakery windows. The sentence would be wrong cause you have to use hand gesture or head nodding to make it a specific one. So that same goes for the other ones :) Hope that was helpful. Feel free to contact me if you need little training.

I will do this in two parts because the second one is way more tricky and I don't want you to get confused: Hello Katha, "The" would cover Der, Das, Die. "Der" is the He form of "The" "Das" is the It form of "The" "Die" is the She form of "The" and also the plural form. Basically what happens is that we Germans give the article a gender to speak frankly. So, for example: Der Baum, The tree. Die Katze, The cat Das Spiel, The game