Introducing sub-topics

Let me begin by explaining why / how ...

First / Now I want to talk about ...

First / Now I want to give you an insight into ...

Let's (now) find out why / how ...

Let's now move to ...

As I already indicated ...

Another aspect / point is that ...

The roots of ... go back to ...

... began when ...

Legend has it that ...

As you probably know, ...

You probably know that ...

Maybe you've already heard about ...

You might have seen that already.

At the beginning there was / were ...

Many people knew / know ...

Hardly anyone knew / knows ...

... hit the idea to ...

... was the first to ...

It is claimed that ...

One can say that ...

I have read that ...

Pictures and graphics

Let me use a graphic to explain this.

The graphic shows that ...

As you can see (in the picture) ...

In the next / following picture, you can see ...

Here is another picture.

The next picture shows how ...

Let the pictures speak for themselves.

I think the picture perfectly shows how / that ...

Now, here you can see ...

Final thoughts on a sub-topic

It was a great success for ...

It is a very important day in the history of ...

It was / is a very important / special event.

This proves that ...

The reason is that ...

The result of this is that ...

It's because ...

In other words, ...

I want to repeat that ...

I'd (just) like to add ...