Si morbo gravi laboras, medicum vocare debes.

Because laborare in the sense of to suffer from or to be afflicted with takes the ablative in Latin.

So:

• morbus gravis = a serious illness
• morbo gravi = with/from a serious illness in the ablative

This is one of those verb patterns you simply learn with the verb:

• morbo laborare = to suffer from an illness
• febre laborare = to suffer from a fever

Even though English uses from or no preposition at all, Latin often uses the ablative alone here.

Originally, laborare is related to to toil, to struggle, or to be in difficulty. In medical contexts, it commonly means to suffer from.

So in this sentence:

• Si morbo gravi laboras = If you are suffering from a serious illness

It does not mean if you work with a serious illness here. The illness phrase morbo gravi shows that the verb is being used in the sense of to be afflicted with.

Because medicum is the direct object of vocare.

• medicus = doctor as the subject form
• medicum = doctor as the object form

Since the sentence says you ought to call a doctor, the doctor is the person being called, so Latin uses the accusative:

• medicum vocare = to call a doctor

If doctor were the subject, you would use medicus instead.

Because after debeo (I ought / I must / I should), Latin normally uses an infinitive to express what someone ought to do.

So:

• debes = you ought / you must
• vocare = to call
• vocare debes = you ought to call / you must call

This is very similar to English:

• You ought to call
• You must call

Other examples:

• legere debeo = I ought to read
• venire debent = they must come

Yes. Latin word order is much freer than English word order because the endings show the grammatical roles.

Here, medicum vocare debes is a very natural order, but Latin could also say:

• debes medicum vocare
• medicum debes vocare

All of these can mean basically the same thing: you should call a doctor.

That said, word order can add emphasis. Putting a word earlier can make it stand out more. In a basic learning sentence like this, the order is just a normal, straightforward Latin arrangement.

This is a simple present condition.

• si = if
• laboras = you are suffering
• debes = you ought / must

So the whole sentence expresses a real or general condition:

• If you are suffering from a serious illness, you should call a doctor.

Latin often uses the present indicative in both parts of this kind of statement when it means something real, straightforward, or generally true.

Because the sentence is addressed directly to you.

Both forms are second person singular:

• laboras = you suffer / you are suffering
• debes = you ought / you must

The ending -s in each verb tells you it is you singular.

If it were talking about he/she, you would get:

• laborat = he/she suffers
• debet = he/she ought

So:

• Si morbo gravi laborat, medicum vocare debet.
  = If he/she is suffering from a serious illness, he/she should call a doctor.

It can mean any of those depending on context.

• must can sound stronger
• should can sound softer
• ought to is often a very good match for debes

In this sentence, debes probably means something like:

• you should call a doctor
• you ought to call a doctor

Since it is medical advice, English might choose should for naturalness, but the Latin verb itself can cover a range from obligation to strong recommendation.

Yes. Si simply means if, and Latin commonly uses it in practical advice like this.

English sometimes says:

• If you have a serious illness, call a doctor
• When you have a serious illness, you should call a doctor

Latin here uses si because it frames the situation as a condition: if this happens, then do that.

Because Latin has no articles like English a, an, and the.

So medicum can mean:

• a doctor
• the doctor

You decide from context which is best. In this sentence, English naturally says a doctor:

• You should call a doctor

But Latin itself does not include a separate word for a.

You would normally look them up like this:

• si = if
• morbus, morbi (masculine) = illness, disease
• gravis, grave = heavy, serious, severe
• laboro, laborare, laboravi, laboratum = to labor, struggle; suffer from
• medicus, medici (masculine) = doctor
• voco, vocare, vocavi, vocatum = to call
• debeo, debere, debui, debitum = to owe; ought; must

This is useful because the sentence contains inflected forms:

• morbo comes from morbus
• gravi comes from gravis
• laboras comes from laboro
• medicum comes from medicus
• vocare comes from voco
• debes comes from debeo