Learn French: Bien sûr, Monsieur ! Je réserve une table. - Of course, sir! I am reserving a table.

Bien normally means "well" and sûr means "sure" or "certain". Put together, bien sûr is a very common set phrase in French that simply means "of course" or "certainly".

How do you actually pronounce Monsieur? It looks like "mon-sieur".

The spelling of Monsieur is historical and doesn't match how it sounds today! It is pronounced roughly like "muh-syuh". The n is silent, and the on sounds more like the "uh" in "cup".

Do you always capitalize Monsieur?

When you are using it as a title or speaking directly to someone, like saying "sir" in English, you capitalize Monsieur. If you are just talking about a man (like "a gentleman"), it is lowercase: un monsieur.

Why does sûr have a little hat accent on the u?

That accent is called a circumflex. In this case, it helps tell the difference in writing between sûr (meaning "sure") and the word sur (meaning "on" or "upon"), which sound exactly the same.

The translation says "I am reserving", but isn't Je réserve just "I reserve"?

In French, there is no separate "-ing" tense (present continuous) like we have in English. The single French present tense je réserve translates to both "I reserve" and "I am reserving" depending on what makes the most sense in English.