Chemistry - Fischer to Haworth projection: which carbon connects to the oxygen?

Solution 1:

It just is. There are pyranose and furanose forms for all aldoses and hexoses (given they are long enough), including glucose:

$\hspace{3cm}$

You will just have to remember which connections correspond to which sugars. You can read more about anomers ($\alpha$ vs. $\beta$) here, and more about carbohydrate chirality (D vs. L) here.

Solution 2:

All sugars attach to carbon n-1 when making pyranose rings, except for the ones with 4 carbon atoms. The reason being, a ring with four atoms wouldn't be stable (too much strain).