Tento plán je mnohem lepší.

Because plán is a masculine singular noun in the nominative case, and Czech demonstratives change form to match the noun.

For example:

• tento plán = this plan
• tato kniha = this book
• toto město = this town/city

So tento is the correct form for a masculine singular noun like plán.

Yes, but the nuance is a little different.

Tento is the clearest, most explicit way to say this.
Ten often means that, but in everyday Czech it can also refer to something already known from context, so it may sound less precise.

For a learner, tento plán is the safest match for this plan.

Je is the 3rd person singular present form of být, meaning to be.

So:

• plán je lepší = the plan is better / this plan is better

In standard Czech, you normally include je in a sentence like this.

Because lepší is the irregular comparative of dobrý.

The pattern is:

• dobrý = good
• lepší = better
• nejlepší = best

This is similar to English good → better → best, which is also irregular.

Czech adjectives do agree with nouns in gender, number, and case, and comparative adjectives can decline too.

However, in the nominative singular, lepší looks the same with different genders:

• lepší plán
• lepší kniha
• lepší město

So the agreement is there grammatically, but in this form you do not see a different ending.

It strengthens the comparative.

• lepší = better
• mnohem lepší = much better / far better

This is a very common Czech pattern:

• mnohem větší = much bigger
• mnohem horší = much worse

Use než.

For example:

• Tento plán je mnohem lepší než ten starý.
• This plan is much better than the old one.

For learners, comparative + než is the most useful basic pattern.

Yes. Czech word order is more flexible than English because endings carry much of the grammar.

Tento plán je mnohem lepší is the neutral, natural order.
If you change the order, the meaning can stay the same, but the emphasis changes. For example:

• Mnohem lepší je tento plán.

This sounds more contrastive or emphatic.

Czech does not have articles like a/an or the.

Definiteness is usually understood from context, or shown by words like tento. So tento plán already clearly points to a specific plan.

The á shows that the vowel is long.

It is not a stress mark. In Czech, stress usually falls on the first syllable of the word, while marks like á mainly show vowel length. So plán has a long a sound.