המועמד שישב לידי אמר שזה התפקיד הכי חשוב שהיה לו עד עכשיו.

Because ה־ is the Hebrew definite article, meaning the.

• מועמד = a candidate / candidate
• המועמד = the candidate

So המועמד שישב לידי means the candidate who sat next to me.

Here ש־ is a very common linking word in Hebrew. It often means that, who, or which, depending on context.

In this sentence:

• שישב לידי = who sat next to me
• שזה התפקיד... = that this/it is the position...
• שהיה לו = that he had

So the same little word ש־ is doing slightly different jobs in English, but in Hebrew it is the normal connector for all of them.

Because the sentence is talking about a completed past action: the candidate sat next to me.

• יושב = sits / is sitting
• ישב = sat

So:

• המועמד שיושב לידי = the candidate who is sitting next to me
• המועמד שישב לידי = the candidate who sat next to me

In your sentence, the past form fits the story.

לידי means next to me / by my side / beside me.

It comes from:

• ליד = next to / beside
• ־י = my / me

So literally it is something like at my side.

Similar forms:

• לידך = next to you
• לידו = next to him
• לידה = next to her

This kind of attached ending is very common in Hebrew.

This is a very natural Hebrew structure.

• אמר ש... = said that...
• זה here helps introduce an identifying sentence, something like it is / this is

So אמר שזה התפקיד הכי חשוב... is a natural way to say:

• He said that it was the most important position...
• or He said that this was the most important position...

In English, it in this kind of sentence can sound a bit empty or structural, and זה often works similarly in Hebrew.

Without זה, the sentence would sound different and less natural in many contexts.

Hebrew usually forms the superlative with הכי + adjective.

So:

• חשוב = important
• הכי חשוב = most important

This is much like saying most + adjective in English.

Examples:

• הכי גדול = biggest / largest
• הכי יפה = most beautiful
• הכי מעניין = most interesting

So התפקיד הכי חשוב = the most important position/job/role.

Because חשוב is describing התפקיד, and תפקיד is a masculine singular noun.

Hebrew adjectives must agree with the noun they describe in gender and number.

• תפקיד חשוב = an important position/role
• משרה חשובה = an important post/job

Even if the person being discussed is male or female, the adjective agrees with תפקיד, not with the person.

This is a classic Hebrew possession pattern.

Hebrew often expresses to have with יש / היה plus ל־ (to someone).

So:

• יש לו = he has
• היה לו = he had

Literally, היה לו is something like there was to him, but idiomatically it simply means he had.

In your sentence:

• התפקיד הכי חשוב שהיה לו = the most important position he had

This is one of the most important structures for English speakers to get used to, because Hebrew does not usually use a direct verb equivalent to English to have in everyday possession sentences.

That ש־ introduces a relative clause describing התפקיד.

So:

• התפקיד הכי חשוב = the most important position
• שהיה לו = that he had

Together:

• התפקיד הכי חשוב שהיה לו = the most important position that he had

In natural English, you might even leave out that:

• the most important position he had

But in Hebrew, using ש־ here is normal.

עד עכשיו means until now, up to now, or so far.

In this sentence, it means that up to the present point in his life or career, this was the most important role/position he had had.

So the sense is:

• the most important position he had had so far

It does not necessarily mean right this second in a strict literal way; it often means up to this point.

It is a little mixed in a way that is very normal in Hebrew.

• זה התפקיד הכי חשוב... is structurally a present-style identifying clause: this/it is the most important position...
• שהיה לו is past: that he had
• The whole thing is inside אמר = said

In English, depending on context, you might translate it as:

• He said that this is the most important position he has had so far
• He said that it was the most important position he had had so far

Hebrew is often less strict than English about sequence of tenses in this kind of sentence.

Yes. תפקיד can mean several related things, depending on context:

• role
• position
• job
• function
• duty

So in this sentence, the best English word depends on the wider context:

• for a company interview: position
• for employment more generally: job
• for responsibilities: role

That is why learners sometimes see slightly different translations for the same Hebrew sentence.

You would change the words that refer to the woman:

• המועמדת instead of המועמד
• שישבה instead of שישב
• אמרה instead of אמר
• שהיה לה instead of שהיה לו

So it would be:

המועמדת שישבה לידי אמרה שזה התפקיד הכי חשוב שהיה לה עד עכשיו.

Notice that חשוב stays masculine, because it still describes תפקיד, which is a masculine noun.