Substitutio alterum heredem ostendit.

You tell mainly from the endings, not from the word order.

• substitutio = the subject
• alterum heredem = the direct object
• ostendit = the verb

So the structure is:

• substitutio — nominative singular, so it is doing the action
• alterum heredem — accusative singular, so it is receiving the action
• ostendit — shows / reveals (depending on context)

That is one of the biggest differences from English: Latin usually marks grammatical roles by case endings.

That is a very normal question. Substitutio is a third-declension noun, not a first- or second-declension one.

Its dictionary forms are:

• substitutio
• substitutionis
  meaning substitution

So:

• nominative singular: substitutio
• genitive singular: substitutionis

Many Latin nouns ending in -tio behave this way. So even though the ending may look unusual at first, substitutio really is nominative singular.

Because alterum is agreeing with heredem.

The dictionary form is alter, altera, alterum, meaning the other (of two) or a second. Here it modifies heredem, so it must match it in:

• gender: masculine
• number: singular
• case: accusative

That gives alterum.

So:

• alter = masculine nominative singular
• alterum = masculine accusative singular

Since heredem is masculine accusative singular, alterum has to be too.

Because heredem is the accusative singular form.

The noun is:

• heres, heredis = heir

Its forms include:

• nominative singular: heres
• accusative singular: heredem

Since it is the direct object of ostendit, Latin uses the accusative: heredem.

So English learners can think:

• heres = subject form
• heredem = object form

Because of agreement.

Alterum is:

• masculine
• accusative
• singular

Heredem is also:

• masculine
• accusative
• singular

But substitutio is:

• feminine
• nominative
• singular

So alterum cannot describe substitutio. It clearly belongs with heredem.

This is a very common Latin pattern: adjectives and similar words match the noun they go with in gender, number, and case.

It can suggest any of those, depending on context.

The key idea of alter is usually:

• the other of two
• or a second one

That is a little more specific than a general another.

English often uses another where Latin might use alter, but Latin can be making a distinction:

• alter = the other one, or a second one
• alius = another, a different one

So in this sentence, alterum heredem suggests something like the other heir or a second heir, depending on the context you were given.

Ostendit is from ostendere, meaning to show, to point out, or to reveal.

In this sentence it is:

• third person
• singular
• active
• indicative

So it means he/she/it shows.

Since the subject is substitutio, the sense is it shows.

A slightly more advanced point: in isolation, ostendit can sometimes be either present or perfect in form when written without extra context. Usually the surrounding context tells you which one is meant. If your learning material gives the present meaning, then that is the intended reading here.

Because Latin does not have articles like English does.

So Latin often leaves it to context whether you should translate a noun as:

• a substitution
• the substitution

or

• an heir
• the heir

That is completely normal. When reading Latin, you often have to supply the or a/an in English based on context.

Yes, this word order is normal.

Latin word order is much freer than English because the endings already show the grammatical roles. This sentence is basically:

• subject: substitutio
• object: alterum heredem
• verb: ostendit

That is a very common Latin arrangement, especially with the verb at the end.

Latin could rearrange the sentence for emphasis, for example:

• Alterum heredem substitutio ostendit
• Substitutio heredem alterum ostendit

Those would still mean essentially the same thing, though the emphasis might shift a little.

Because Latin finite verbs agree with their subject in person and number, not in gender.

So with substitutio the verb is singular because the subject is singular:

• substitutio ostendit = the substitution shows

It does not matter that substitutio is feminine. Gender would matter for adjectives or participles, but not for a normal finite verb like ostendit.