Považuji tento plán za dobrý.

Because Czech usually does not need subject pronouns when the verb form already makes the subject clear.

Here, považuji means I consider, so já is already understood. You could say Já považuji tento plán za dobrý, but that would usually add emphasis, for example:

• Já považuji tento plán za dobrý. = I consider this plan good.
  (maybe implying someone else does not)

So the version without já is the normal neutral one.

Považuji is the 1st person singular present tense of považovat.

So it means:

• I consider
• I regard
• I take ... to be

A few useful details:

• infinitive: považovat
• person: 1st person singular
• tense: present
• aspect: imperfective

In this sentence, it is a normal present-tense statement.

Tento plán is the direct object of považuji, so it is in the accusative case.

The reason it looks unchanged is that plán is masculine inanimate singular, and for many masculine inanimate nouns, the accusative singular is the same as the nominative singular.

So:

• nominative: tento plán
• accusative: tento plán

That is why you do not see a visible ending change here.

Here za is part of the fixed pattern used with považovat.

The structure is:

• považovat něco za něco / za nějaké

So:

• Považuji tento plán za dobrý.

literally works like:

• I consider this plan as good
• more natural English: I consider this plan good / I consider this plan to be good

So za is not optional here in this construction. It helps introduce what something is being considered as.

Because after považovat něco za ..., the word after za is in the accusative, not the instrumental.

Also, dobrý is an adjective, because it describes plán. It is not an adverb, so dobře would be wrong here.

Why does dobrý not visibly change? Again, because it agrees with plán, which is masculine inanimate singular, and in that form the accusative looks the same as the nominative:

• nominative masculine inanimate singular: dobrý
• accusative masculine inanimate singular: dobrý

You can see the change more clearly with other genders:

• Považuji tu knihu za dobrou.
• Považuji Petra za dobrého.

No, that is not natural Czech.

An English speaker may want to copy consider something to be ..., but Czech normally does not use být after považovat like that.

Use one of these instead:

• Považuji tento plán za dobrý.
• Myslím, že tento plán je dobrý.

So with považovat, the normal pattern is:

• považovat + object + za + adjective/noun

not považovat + object + být + adjective.

Yes. Czech word order is flexible, and changing it usually changes the focus or emphasis, not the core meaning.

For example:

• Považuji tento plán za dobrý.
  Neutral.

• Tento plán považuji za dobrý.
  Emphasis on this plan.

• Za dobrý považuji tento plán.
  Stronger emphasis on good or a more marked style.

So the original sentence is a very natural neutral version.

Yes, in many situations you could, but the nuance changes a little.

• tento plán = this plan, more explicit
• ten plán = often that plan, but in everyday Czech it can also function a bit like the/this, depending on context

So:

• Považuji tento plán za dobrý.
  clearly points to this specific plan

• Považuji ten plán za dobrý.
  also possible, often more colloquial/context-dependent

In careful or textbook-style Czech, tento is the clearer choice for this.

Czech does not have articles like English the and a.

That means Czech usually shows definiteness through:

• context
• word order
• demonstratives such as ten or tento

In this sentence, tento already makes the noun specific, so it clearly means this plan.

If you said:

• Považuji plán za dobrý.

that could mean I consider the plan good or I consider a plan good, depending on context.

It is basically neutral standard Czech.

A few style notes:

• považuji is standard and natural
• tento sounds a bit more explicit and sometimes slightly more formal or written than everyday tenhle
• the whole sentence would fit well in speech or writing

A more colloquial way might be something like:

• Myslím, že ten plán je dobrý.

But the original sentence is perfectly normal and correct.