I have some difficults perceiving the differences between sounds very similars (to my ears). For example words like bad and bed sounds very similar. Live and leave too. Though listenig cautiously I can hear the differences between them. But the TH sound (both of them) is very difficult, because sometimes I can perceive them clearly, when they come in the end or sometimes in the middle. For example teeth, health, father, weather (most of time).
The main "problem" is when the TH is not produced in the canonical form and sounds like a approximant.
I am referring as the canonical TH sound that one that continuously flow air and sounds like S and Z said with a lisp effect.
For example, when native says teeth, most of time this final unvoiced TH sounds as a S with a lisp (to my native ears). But other times this sounds to my ears as TEET.
Now my main doubt is that some words are not pronounced in the canonical form. Usually when they begins with TH. For example, THen, THan, THat sound like Den, Dan and Dat. And THink and THought sound (in these cases I can hear exceptions sometimes) like Tink and Tought. For example I can exchange the soft TH sound for T in my language and it sounds the same to me.
My questions are:
1.Does these words sound this way just to my ears or even for natives is possible to cause trouble?
2.Is always possible to hear the difference if someone says That and Dat? Even if this person says the TH in the approximant form?
3.Is considered acceptable saying both TH in the canonical form most of time ( for example"a thing there" would sound like "a sing zere" but putting the lisping effect in S and Z)?