Slyším tě dobře, ale nevidím tě z okna.

Tě is the unstressed object form (accusative) of ty (you, singular, informal).

• ty = you (subject form, nominative)
  
  • Ty slyšíš mě. – You hear me.
• tě = you (object form, accusative/genitive, unstressed)
  
  • Já slyším tebe / tě. – I hear you.

In this sentence:

• Slyším tě dobře – I hear you well (you = direct object)
• nevidím tě z okna – I don’t see you from the window (again, object)

So tě is used because “you” is the object of the verbs slyšet and vidět, not the subject.

You can drop the second tě, but it becomes less clear.

• Slyším tě dobře, ale nevidím tě z okna.
  Very clear: “I hear you well, but I don’t see you from the window.”

• Slyším tě dobře, ale z okna tě nevidím.
  Also clear and natural; just a different word order.

• Slyším tě dobře, ale nevidím z okna.
  Grammatically OK, but now it just says: “I hear you well, but I don’t see (anything) from the window.”
  The listener will probably infer it’s still about them from context, but the sentence no longer explicitly says you.

So for learners, it’s safer and clearer to repeat tě.

Czech uses special “short” pronouns called clitics (like tě, se, si, mi, mě, etc.) that usually can’t stand at the very beginning of a main clause.

They tend to appear in the second position in the clause, often right after the first stressed word (often the verb if it’s first):

• Slyším tě dobře.
• Já tě slyším dobře.
• Dobře tě slyším.

In your sentence, the simplest neutral word order is:

• Slyším tě dobře… – verb + clitic pronoun + adverb

Placing tě before the verb (*Tě slyším dobře) is possible only in some special emphatic contexts and usually with the full form tebe:

• Tebe slyším dobře, ale jeho ne. – You I hear well, but not him.

For everyday neutral speech, verb + tě is what you want.

Czech usually drops subject pronouns when they are obvious from the verb ending.

• Slyším already tells us the subject is já (I), because of the -ím ending (1st person singular, present).
• Nevidím likewise shows I don’t see.

So:

• Já slyším tě dobře, ale já nevidím tě z okna. – possible, but sounds heavy and emphatic.
• Slyším tě dobře, ale nevidím tě z okna. – normal, natural Czech.

You only add já for contrast or strong emphasis:

• Já slyším tě dobře, ale on tě neslyší.
  I hear you well, but he doesn’t.

Dobře is an adverb (well).
Dobrý is an adjective (good).

In English you say “I hear you well” (adverb), not “I hear you good” (adjective). Same in Czech:

• dobrý – describes a noun:
  
  • dobrý člověk – a good person
  • dobrý den – good day / hello
• dobře – describes a verb (how something is done):
  
  • mluví dobře česky – he/she speaks Czech well
  • slyším tě dobře – I hear you well

So after slyším you need the adverb dobře.

Z okna literally means “from (out of) the window”.

• z = from, out of
• okno = window (nominative singular)
• okna = genitive singular form of okno

The preposition z always requires the genitive case, so:

• z okna – from the window
• z domu – from the house
• z práce – from work

In this sentence, nevidím tě z okna means something like:

• I don’t see you from (the vantage point of) the window.

You wouldn’t use od here; od is more like “from (someone/some place as a source or origin)”:

• dostal jsem to od kamaráda – I got it from a friend
• klíče jsou od dveří – the keys are for the door / belong to the door

For “seeing from a viewing point”, z is correct: z okna, z balkónu, z terasy.

• tě is accusative singular of ty (informal “you”)
  
  • used as the direct object of slyším and nevidím
• okna is genitive singular of okno
  
  • required by the preposition z: z + genitive

So:

• Slyším koho? co? → tě (accusative)
• z čeho? z koho? → z okna (genitive after z)

Both involve sound, but they’re not the same:

• slyšet = to hear (passive perception)
  
  • Slyším tě. – I hear you. (I can hear your voice.)
• poslouchat = to listen (to) (active, intentional)
  
  • Poslouchám tě. – I am listening to you. / I obey you.
    (context decides which meaning)

In the sentence:

• Slyším tě dobře = I can hear you well (your volume/clarity is fine)

If you said:

• Poslouchám tě dobře

it would more likely mean I obey you well or I follow your instructions well, not about audio clarity.

Yes. Czech word order is fairly flexible, especially for adverbs and prepositional phrases. All of these are possible and natural, with only slight changes in emphasis:

• Slyším tě dobře, ale nevidím tě z okna.
  Neutral.

• Dobře tě slyším, ale z okna tě nevidím.
  Slight emphasis on “well” and on the contrast “from the window”.

• Tě slyším dobře, ale z okna tě nevidím.
  Here tě is emphasized; you’d use this when contrasting you with someone else.

The main restrictions:

• The clitic tě normally shouldn’t be at the very beginning of the clause.
• Keep z okna close enough to nevidím, so it’s clear that “from the window” belongs to the seeing, not the hearing.

Czech often expresses inability simply by negating the main verb, without adding moci (to be able to).

So:

• Nevidím tě z okna. – Literally: I don’t see you from the window, but in context it usually corresponds to “I can’t see you from the window.”
• Neslyším tě. – I don’t hear you / I can’t hear you.

You can say:

• Nemůžu tě vidět z okna. – I cannot see you from the window.

This adds a bit more explicit sense of impossibility, but in many everyday contexts, nevidím tě is enough and perfectly natural.

In Czech, ale (but) usually joins two independent clauses, and a comma is required:

• Slyším tě dobře, ale nevidím tě z okna.

Each part could stand as its own sentence:

• Slyším tě dobře.
• Nevidím tě z okna.

That’s why we put a comma before ale. This is very similar to English punctuation here.

Basic rules:

• Czech stress is almost always on the first syllable of each word.
• Vowels written with an accent (á, é, í/ý, ó, ú/ů) are long.

So:

• Sly-ším – SLY-šim (stress on Sly-; í is long: slee-)
• tě – one syllable, ťe (palatalized ť, close to tye)
• dob-ře – DOB-ře (ř is the Czech “r+ž” sound)
• a-le – A-le
• ne-vi-dím – NE-vi-dím (í long again)
• z ok-na – Z OK-na (stress on ok, because z is a preposition and usually unstressed)

Put together slowly:

SLY-šim tě DOB-ře, a-le NE-vi-dím tě z OK-na.

You’d switch from informal ty / tě to formal vy / vás:

• Slyším vás dobře, ale nevidím vás z okna.

Details:

• vy – you (formal, subject)
• vás – you (formal, object, both accusative and genitive)

Everything else (verbs, word order, z okna, dobře) stays the same.