Exchanging TH (voiced) sound with D sound?

Question

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)?

Answer 1

All feedback in the comments is very good. To the native speakers, these sounds pose no ambiguity.

Additionally, I can attest that even for a non-native speaker, these sounds are not an issue of confusion, as long as the other person uses / creates them (the sounds) properly.

I can also attest that I cannot understand the subtle differences in speech in other languages, and colleagues make (friendly) fun of me. The same happens when I try to pronounce some words - regardless of their patience, I cannot (yet) get it right.

Currently I try to help colleagues (in Slavic country) to improve their English level. The strong conclusion that I reached was that "problems" of listening and speaking are actually problems of training.

What I mean: if you speak (a lot) and listen (a lot) to proper English, you will get to the point to understand the subtle differences with no effort.

Conclusion: the key to get rid of this problem is practice.

My kind advice:

• listen as much as possible native English speaking. If you do not have friends, use movies. Some movies are more helpful, others less, depending on how they speak English. It can be less helpful if they speak fast and / or with specific strong accents.
• practice to pronounce the words as correct as possible. Do not simplify by using "s" or "z" or "d" instead of "th". If you simplify, you only delay the moment when you will speak good English.