[Grammar] pronunciation th vs th

[rolland04]

in english phonetics, and surprisingly for non-native speakers, there are two different sounds for a "th" spelling, marked by a different phonetic symbol.
Is there any rule of thumb about which one is when ?

 

[Raymott]

The small common function words are all the soft 'th' - the, there, they, them, that, this, their, these, those ...