No sabes lo tranquila que me siento cuando la dentista me explica todo con calma.

Not always literally. In sentences like this, No sabes... is often used as an emphatic expression, similar to:

• You have no idea...
• You don't know how...
• You can't imagine...

So No sabes lo tranquila que me siento... means something like You have no idea how calm I feel...

It is a very natural way in Spanish to introduce a strong feeling or reaction.

This is a very common Spanish structure for emphasis:

• lo + adjective + que

It means how + adjective in exclamatory or emphatic English.

So:

• lo tranquila que me siento = how calm I feel

Other examples:

• No sabes lo difícil que fue = You don't know how difficult it was
• Mira lo bonito que es = Look how beautiful it is
• No imaginas lo cansado que estoy = You can't imagine how tired I am

In this structure, lo does not mean it. It is part of an intensifying pattern.

Because the adjective agrees with the person who feels that way.

In me siento, the speaker is talking about I feel. If the speaker is female, she says:

• me siento tranquila

If the speaker is male, he would usually say:

• me siento tranquilo

So the full sentence would change to:

• No sabes lo tranquilo que me siento...

if the speaker were male.

Because sentirse means to feel in the sense of to feel emotionally or physically in a certain state.

• me siento tranquila = I feel calm

Here, me is the reflexive pronoun that goes with sentirse.

Compare:

• siento miedo = I feel fear
• me siento tranquila = I feel calm

So:

• sentir + noun: siento alegría, siento dolor
• sentirse + adjective/adverbial description: me siento bien, me siento tranquila

That me is different from the one in me siento.

In la dentista me explica todo, me means to me:

• la dentista = the dentist
• me explica = explains to me
• todo = everything

So:

• La dentista me explica todo = The dentist explains everything to me

Spanish often uses an indirect object pronoun like me, te, le, nos, os, les even when English may not.

Yes, dentista often has the same form for both genders, but the article changes:

• el dentista = the male dentist
• la dentista = the female dentist

In your sentence, la dentista tells us the dentist is female.

This is common with some profession words ending in -ista:

• el artista / la artista
• el turista / la turista
• el dentista / la dentista

Both are possible, but they are slightly different in tone.

• la dentista = the dentist
• mi dentista = my dentist

Using la dentista can sound more neutral or general, while mi dentista makes it more personally specific.

So the sentence could also be:

• No sabes lo tranquila que me siento cuando mi dentista me explica todo con calma.

That would also be perfectly natural.

Because the sentence describes something that happens regularly or whenever that situation occurs:

• cuando la dentista me explica todo con calma = when the dentist explains everything to me calmly

This use of the present tense with cuando is normal when talking about repeated or general situations.

It is like saying:

• Whenever the dentist explains everything calmly, I feel very calm.

If you were talking about one specific past occasion, you would normally use a past tense instead:

• cuando la dentista me explicó todo con calma

Con calma means calmly, slowly, without rushing, or in a calm way, depending on context.

Here it suggests the dentist explains things in a reassuring, patient manner.

So:

• me explica todo con calma = she explains everything to me calmly / patiently

It is very common in Spanish to use con + noun where English might prefer an adverb:

• hablar con claridad = to speak clearly
• responder con paciencia = to answer patiently
• explicar con calma = to explain calmly

Grammatically, yes, but con calma is much more natural here.

• explica todo con calma sounds everyday and idiomatic.
• explica todo calmadamente is understandable, but it sounds much less common and more formal or bookish.

In many cases, Spanish prefers a phrase like con calma instead of an -mente adverb.

Spanish often uses structures that do not match English word-for-word.

The pattern here is:

• lo + adjective + que + verb

So:

• lo tranquila que me siento

Literally, it is something like the calm that I feel, but that is not how it works in real English. The natural translation is:

• how calm I feel

This is one of those structures that is best learned as a chunk rather than translated word by word.

Yes, it is very natural in conversation. It sounds expressive, but not unnatural.

It is the kind of thing someone might say when talking about a doctor, teacher, friend, or partner who makes them feel reassured.

Similar everyday expressions are:

• No sabes lo feliz que estoy
• No sabes lo nervioso que me puse
• No sabes lo bien que explica

So this sentence sounds warm, natural, and conversational.

Not always. It depends on whether you are talking about:

1. habitual/repeated actions
   -> usually present tense

   • Cuando la dentista me explica todo con calma, me siento mejor.

2. past events
   -> past tense

   • Cuando la dentista me explicó todo con calma, me relajé.

3. future situations
   -> often present subjunctive after cuando

   • Cuando la dentista me explique todo, me tranquilizaré.

So in your sentence, the present tense is used because it describes a usual or repeated situation.