Learn Hardcore Hausa: Shugaba yana da muhimmanci sosai a al'umma. - A leader is very important in the community.

So word‑for‑word you get something like: “A leader has importance very in society.”
Natural English: “A/The leader is very important in society.”

Hausa often expresses “to be X (an adjective)” by saying “to have X‑ness (a noun)”:

yaro yana da tsawo – the boy is tall (lit. has tallness)
shugaba yana da muhimmanci – the leader is important (lit. has importance)

Why is there no word for the before Shugaba? How do you say the leader vs a leader?

Hausa has no separate words for “the” or “a”. A bare noun like shugaba can mean “a leader” or “the leader” depending on context.

If you want to make “a specific, known leader” clearer, you normally add more information:

shugaban ƙasa – the president (leader of the country)
shugaban makaranta – the head teacher / principal
shugaban ƙungiya – the leader of the association

In your sentence, Shugaba yana da muhimmanci sosai a al'umma can be translated either as “A leader is very important in society” (general statement) or “The leader is very important in society” (if a specific leader is understood from context).

What is the difference between muhimmanci and muhimmi?

muhimmi = important (adjective)
muhimmanci = importance (noun, “the quality of being important”)

Your sentence uses the noun:

yana da muhimmanci – he has importance / he is important

You can also use the adjective directly in other patterns:

Shugaba muhimmi ne a al'umma. – The leader is important in society.
Wannan abu muhimmi ne. – This thing is important.

So muhimmanci works well after yana da, while muhimmi often appears with ne/ce or in phrases like abu muhimmi (“an important thing”).

Where should sosai go in the sentence? Could it come before muhimmanci or at the beginning?

sosai is an intensifier meaning “very / really / extremely”, and it normally comes after the verb or adjective/quality it modifies.

Natural positions:

Shugaba yana da muhimmanci sosai a al'umma.
Yana da muhimmanci sosai.

Putting sosai before muhimmanci (*sosai muhimmanci) or at the very start (*Sosai shugaba…) is not how Hausa normally does it. Keep sosai after muhimmanci (or after another adjective/verb) to sound natural.

What exactly does a al'umma mean? Could I say a cikin al'umma or ga al'umma instead?

a is a basic preposition meaning “in / at / on”.
al'umma means “community / society / people (as a social group)”.

So a al'umma ≈ “in society / in the community.”

Other options:

a cikin al'umma – literally “in the inside of society”, often translated “within society”; a bit more explicit but very common and natural.
ga al'umma – “to / for society”, used more like “important to society / for society” (emphasising benefit or relevance).

All of these are possible, with small nuances:

muhimmanci … a al'umma – importance in society (position/role).
muhimmanci … ga al'umma – importance to/for society (benefit).
muhimmanci … a cikin al'umma – importance within society (inside the social structure).

How do you pronounce al'umma, and what does the apostrophe represent?

The apostrophe in al'umma marks a glottal stop – a brief catch or stop in the throat, like the break in the middle of “uh‑oh” in English.

Pronunciation (simplified):

al'umma ≈ al [stop] um‑ma

You do not blend the a and u together. Instead, you slightly pause between them:

a
- (glottal stop)
  - lumma

So a al'umma sounds roughly like: a al‑[stop]um‑ma.

Can Shugaba refer to any kind of leader, or only a president?

Shugaba is a general word for “leader / head / chief / chairperson.” It is not limited to “president”.

Examples:

shugaban ƙasa – president (of a country)
shugaban kamfani – company director / CEO
shugaban ƙungiya – leader of an association / group
shugaban gida – head of the household

In your sentence, Shugaba can mean “a leader” in a broad sense, not necessarily a political president.

Why don’t we need a pronoun like shi in this sentence? Would Shi yana da muhimmanci sosai a al'umma be correct?

In Hausa, the subject is already shown in the verb form, so you usually do not add a separate pronoun unless you want to emphasize it.

yana da – he has / he is having (3rd person singular masculine)
Shugaba yana da… – The leader has… (subject expressed by Shugaba)

Adding shi in front:

Shi yana da muhimmanci sosai a al'umma.

is grammatically possible, but it sounds like emphasis or contrast, for example:

Shi yana da muhimmanci sosai, amma wani ba haka ba.
He is very important, but the other one is not.

For a neutral statement, Shugaba yana da… (without shi) is the natural form.

How would you say this in the plural: “Leaders are very important in society”?

The plural of shugaba is shugabanni (leaders), and the plural of yana is suna:

Shugabanni suna da muhimmanci sosai a al'umma.

This means “Leaders are very important in society.”
You can also say:

Shugabanni suna da muhimmanci sosai a cikin al'umma. – Leaders are very important within society.

How can I say the same idea in the past tense: “The leader was very important in society”?

A natural way is to use ya kasance (“was / used to be”) plus a phrase meaning “important”:

Shugaba ya kasance mai muhimmanci sosai a al'umma.
The leader was very important in society.

Breakdown:

ya kasance – he was
mai muhimmanci – (one) having importance / important
sosai – very
a al'umma – in society

You could also say:

Shugaba ya yi matuƙar muhimmanci a al'umma.
(literally: The leader did great importance in society, i.e. played a very important role in society.)