To begin with, I believe
At first sight, I suppose
In the first place, I think That
As a matter of fact, I guess
To start with, I would like to say
First (of all), firstly...

To begin with, I would like to say that learning a foreign language is very important.

ADDITION: Besides, Moreover, Furthermore, In addition, What is more,...

CONTRAST: However, Nevertheless, But, Although, On the one hand, on the other hand, In contrast, while, when, whereas...

GIVE OPINION: In my opinion, from my point of view, As far as I am concerned, I (don't) (strongly) believe that...

AGREE/DISAGREE: I agree with/to, I don't agree with/to, I disagree...

SEQUENCE: First, firstly, secondly, thirdly... then, later, next... after, afterwards, after that, as soon as... finally...

CAUSE/EFFECT: because, as/since, so/therefore, as a result, this is why, for this reason, due to/beacuse of, consequently

GIVE EXAMPLE: for example, for instance, like, apart from, such as...

To conclude
To sum up I feel
In short I understand
In all It is clear that
In brief I agree/disagree
In conclussion
Summing up