Learn English – How to guess if some letter in some word is silent or not

There are several reasons, three of which stand out:

1. If you keep the original spelling, connections with the same word used in other languages remain intact. In honest and psychology, for example, this makes it easier for people to communicate across English, French, Latin, Greek, etc. In German, they spell Zentrum. This makes it harder for us to remember how it is spelled, and sometimes this even makes it harder to learn the word for learners. Similarly, Dutch spells akkoord, which is again less consistent with many other languages and the language of origin, French.

2. It is terribly hard to get people to change the way they have always spelled words. Imagine if you had to change your spelling all of a sudden after having spelled a certain word the same way for 30 years! Or, what may be even worse, regular spelling changes occurring every 5 years, which nobody will be able to learn or keep up with, as in Dutch. And, even if you decide that something needs to change, there will be 1001 opinions as to how. Should it be onest? onnest? onnist? As it is, no spelling system in the world is wholly phonetic, not even new systems. Imagine you had to read a teks laik this, weh ai'uhv tsyowsin suhtin konvensyuhns.

3. How will you take into the account the fact that people in different regions pronounce the same word differently? Should all spelling be based on the phonetics of Received Pronunciation? Or should each region use its own spelling, making, say, British harder to read for Australians and vice versa?

In general, there is no rule in English to tell you when to pronounce the 'h', but it is rarely silent. That seems to answer the superficial reading of your question. So you have to learn the exceptions as they come along. Usually they turn out to borrowings from French, like 'honest'

But your examples seem to be about 'th'. In general, in English, 'th' is almost always pronounced as a single sound, the lisping tongue-between-the-teeth hiss. So you have to learn the exceptions as they come along. For your two examples, they are both borrowings from foreign languages and spellings are decided on arbitrarily, sometimes based on the original spelling (as in the case of German) or by accepted correspondence between the writing of the original language and English (or sometimes someone just makes it up and it sticks).

Since the 'th' sound is rare in all the languages of the world, and English is one of the few with that sound, the rule of thumb for you would be, if reading English, if you see a 'th' spelling, if it looks non-English, it's probably pronounced like 't', but if it looks native English, it's probably the lisping sound.