Der Gärtner erklärt, man müsse mit dem Spaten vorsichtig graben, damit die Wurzeln ganz bleiben.

müsse is Konjunktiv I (subjunctive I), which is very commonly used in German for reported speech or indirect speech.

So the sentence is not just giving the gardener’s words directly; it is presenting what he explains or says:

• direct style: Man muss mit dem Spaten vorsichtig graben.
• indirect style: Der Gärtner erklärt, man müsse mit dem Spaten vorsichtig graben.

Using müsse shows that this is the gardener’s statement being reported.

In everyday spoken German, many people would also say:

• Der Gärtner erklärt, dass man mit dem Spaten vorsichtig graben muss.

That is more conversational. The version with müsse sounds a bit more formal or written.

man in German means one, people, or you in a general sense.

It does not mean man as in an adult male.

So here man müsse ... graben means something like:

• one must dig carefully
• you have to dig carefully
• people need to dig carefully

It is used when the statement is general and not aimed at one specific person.

The preposition mit always takes the dative case, so der Spaten becomes dem Spaten.

• nominative: der Spaten
• dative: dem Spaten

So:

• mit dem Spaten = with the spade

This is a very common thing to memorize:
mit + dative

Here vorsichtig is an adverb, not an adjective describing a noun.

It modifies the verb graben and tells you how the digging should be done:

• vorsichtig graben = to dig carefully

Adverbs in German do not get adjective endings, so vorsichtig stays the same.

Compare:

• ein vorsichtiger Gärtner → adjective describing a noun
• vorsichtig graben → adverb describing the action

Because müsse is a modal verb, and modal verbs usually send the main verb to the end in the infinitive.

So in man müsse mit dem Spaten vorsichtig graben:

• müsse = the finite modal verb
• graben = the main verb in the infinitive

This is the normal pattern:

• man muss graben
• man müsse graben
• dass man graben muss

The exact placement of the finite verb changes depending on the clause type, but the infinitive graben stays at the end.

The comma separates the main clause from the clause that contains what the gardener explains.

• main clause: Der Gärtner erklärt
• reported statement: man müsse mit dem Spaten vorsichtig graben, damit die Wurzeln ganz bleiben

German uses commas very regularly to separate clauses, often more consistently than English does.

Here damit means so that or in order that.

It introduces a purpose clause:

• ..., damit die Wurzeln ganz bleiben.
• ..., so that the roots stay intact.

The idea is: dig carefully for the purpose of keeping the roots undamaged.

A useful contrast:

• weil = because
• damit = so that / in order that

So this clause is not giving the reason why the roots stay intact; it gives the purpose of digging carefully.

Because damit introduces a subordinate clause, and in German subordinate clauses normally send the finite verb to the end.

So:

• die Wurzeln bleiben ganz → main clause order
• damit die Wurzeln ganz bleiben → subordinate clause order

This verb-final pattern happens with many subordinating conjunctions, such as:

• weil
• dass
• wenn
• obwohl
• damit

ganz bleiben is a very natural German expression meaning:

• to remain whole
• to stay intact
• not to get damaged/broken

So damit die Wurzeln ganz bleiben means the roots should remain unbroken or undamaged.

German often uses combinations like this:

• kaputt gehen = to break / stop working
• sauber bleiben = to stay clean
• ruhig bleiben = to stay calm
• ganz bleiben = to stay intact

So this is a normal and idiomatic way to say it.

die Wurzeln is nominative plural here because it is the subject of bleiben.

The roots are the thing that stays intact:

• die Wurzeln bleiben ganz

Because Wurzeln is plural, the verb is also plural:

• die Wurzeln bleiben
• not die Wurzeln bleibt

Yes. A very common alternative is:

Der Gärtner erklärt, dass man mit dem Spaten vorsichtig graben müsse, damit die Wurzeln ganz bleiben.

Or in more everyday German:

Der Gärtner erklärt, dass man mit dem Spaten vorsichtig graben muss, damit die Wurzeln ganz bleiben.

These versions are all possible, but they differ a bit in style:

• ..., man müsse ... → concise, formal reported speech
• ..., dass man ... müsse → also formal, explicit subordinate clause
• ..., dass man ... muss → very common in everyday German

It sounds somewhat formal or written, mainly because of müsse in indirect speech.

A native speaker would understand it easily, but in casual spoken German many people would more naturally say something like:

• Der Gärtner sagt, dass man mit dem Spaten vorsichtig graben muss, damit die Wurzeln ganz bleiben.

So the original sentence is perfectly normal German, but it has a slightly more polished or report-like tone than relaxed conversation.