Learn Hardcore German: Die Wissenschaftlerin forscht seit Jahren zu diesem Thema. - The scientist female has been doing research on this topic for years.

Wissenschaftlerin is the explicitly female form of Wissenschaftler.
- der Wissenschaftler = (male) scientist / generic “scientist”
- die Wissenschaftlerin = female scientist
The suffix -in (singular) and -innen (plural) is added to many profession nouns to form the feminine:
- der Lehrer → die Lehrerin
- der Arzt → die Ärztin
- der Wissenschaftler → die Wissenschaftlerin

Because the subject here is a woman, the sentence uses the feminine noun Wissenschaftlerin with the feminine article die.

What exactly does the verb forscht mean, and how is it different from recherchiert or studiert?

forscht is the 3rd person singular form of forschen.

forschen = to do research, to conduct research, usually in a scientific or scholarly sense.
- Die Wissenschaftlerin forscht zu diesem Thema.
  → The scientist is doing research on this topic.

Comparison:

recherchieren = to research in the sense of looking up information, e.g. for a newspaper article, a school report, or online.
- Der Journalist recherchiert den Fall.
studieren = to study (at university) or to study something in depth.
- Sie studiert Physik. = She studies physics (at university).

So forschen is about carrying out systematic, often academic or scientific, research, while recherchieren is more about gathering information and studieren about studying a subject.

Why is the verb in the simple present (forscht) even though English uses “has been researching”?

German often uses the simple present with seit (“for/since”) to express an action that started in the past and continues into the present.

Die Wissenschaftlerin forscht seit Jahren zu diesem Thema.
Literally: The scientist researches since years on this topic.
Natural English: The scientist has been researching this topic for years.

So:

seit + present tense in German ≈ “has/have been …-ing for/since …” in English.

You can also see:

Die Wissenschaftlerin hat seit Jahren zu diesem Thema geforscht.

But with seit and an ongoing action, the present tense (forscht) is more typical and sounds more neutral and natural in standard German.

What does seit Jahren literally mean, and how is it different from seit vielen Jahren or jahrelang?

Literally:

seit Jahren = “since years”, but idiomatically it means “for years”.

Nuances:

seit Jahren
→ for years (a long, unspecified time; neutral)
seit vielen Jahren
→ for many years (emphasises that it’s really a lot of years)
jahrelang (adverb)
→ for years on end / for years and years (often with a stronger emotional tone, like a very long time).

All can describe a long duration; the choice is about emphasis rather than grammar.

Why do we use zu diesem Thema here? Could we also say über dieses Thema or an diesem Thema?

With forschen, several prepositions are possible, each with a slightly different nuance:

zu diesem Thema
- Very common in academic / formal contexts.
- Focus on the research topic/field.
- Feels a bit like “on this topic” in academic English:
  She conducts research on this topic.
über dieses Thema
- More like “about this topic”.
- Focus on the content she is investigating or writing about.
- Sie forscht über dieses Thema. is also correct and natural.
an diesem Thema
- Literally “on this topic”, but nuance is closer to “working on this topic/project”.
- Often used when the topic is seen as a project, problem, or object you work on:
  Sie forscht an einem Heilmittel. = She is researching a cure.

In this sentence, zu diesem Thema is perfectly standard; über dieses Thema and an diesem Thema would also be grammatically correct, just with slightly different emphasis or style.

What case is diesem Thema, and how can I recognize it?

diesem Thema is dative singular neuter.

Step by step:

The base noun is das Thema (neuter).
The preposition zu always takes the dative case.
Dative singular of dies- (this) is:
- masculine: diesem
- neuter: diesem
Dative singular of das Thema is dem Thema (the noun itself doesn’t change form).

So:

zu + dem Thema → zu dem Thema
Combined with dies-: zu diesem Thema

That’s why we get diesem Thema, not dieses Thema.

Why is it diesem Thema and not dieses Thema?

Because of the preposition and the case:

zu → always requires dative.
dieses is accusative or genitive neuter, not dative.
- accusative neuter: dieses Thema (e.g. über dieses Thema)
- dative neuter: diesem Thema

Compare:

zu diesem Thema (dative, because of zu)
über dieses Thema (accusative, because of über here)

So in this sentence diesem is the correct dative form after zu.

Can I say Die Wissenschaftlerin forscht dieses Thema? Why or why not?

No, that is not correct German.

forschen is used without a direct accusative object in this meaning.
You cannot say etwas forschen like you say research something in English.
Instead, you use a prepositional phrase:
- zu diesem Thema forschen
- über dieses Thema forschen
- an diesem Thema forschen

Think of forschen more like “to do research (on something)”, where the “on something” must be expressed with a preposition in German.

Can the word order change, for example Seit Jahren forscht die Wissenschaftlerin zu diesem Thema?

Yes, German word order is flexible for these adverbials, as long as the conjugated verb stays in second position in a main clause.

All of these are correct:

Die Wissenschaftlerin forscht seit Jahren zu diesem Thema.
Die Wissenschaftlerin forscht zu diesem Thema seit Jahren.
Seit Jahren forscht die Wissenschaftlerin zu diesem Thema.

Nuances:

Putting Seit Jahren at the beginning (3) emphasizes how long she has been researching.
Putting zu diesem Thema earlier emphasizes the topic a bit more.

Grammatically, they’re all fine; it’s mostly a matter of emphasis and style.

How would I say this sentence if the research is finished now?

If the research has ended, you normally do not use seit, because seit implies it still continues.

Instead, you show duration with something like jahrelang or a time phrase, and use a past tense:

Die Wissenschaftlerin forschte jahrelang zu diesem Thema.
→ The scientist researched this topic for years. (Simple past, especially in writing.)
Die Wissenschaftlerin hat jahrelang zu diesem Thema geforscht.
→ The scientist researched this topic for years. (Present perfect, very common in spoken German.)

If you want to show when it ended:

Die Wissenschaftlerin hat bis 2020 jahrelang zu diesem Thema geforscht.
→ She researched this topic for years until 2020.

How do you pronounce Wissenschaftlerin and forscht?

Approximate pronunciation in Standard German:

Wissenschaftlerin
- IPA: [ˈvɪsənʃaftləʁɪn]
- Rough English approximation: VI-sen-shaft-le-rin
  - W sounds like English v.
  - sch = English sh.
  - Stress is on the first syllable: WIssenschaftlerin.
forscht
- IPA: [fɔʁʃt]
- Rough English approximation: for-sht (one syllable):
  - r is the German R (often uvular in Standard German, like a throat sound).
  - sch again = sh.
  - Final t is clearly pronounced.

Together:
Die Wissenschaftlerin forscht seit Jahren zu diesem Thema.
≈ Dee VI-sen-shaft-le-rin for-sht zite YA-ren tsu DEE-zem TAY-ma.

Is Wissenschaftlerin the same as “researcher,” “scientist,” or “academic”?

Wissenschaftlerin covers several of these ideas, depending on context:

Basic meaning: female scientist / scholar / academic.
In German, Wissenschaft includes not only natural sciences but also humanities and social sciences.
- A philosopher or historian at a university can also be a Wissenschaftler / Wissenschaftlerin.

Closest English equivalents:

In many contexts: scientist or researcher (female).
In a university context: often academic or scholar can also fit.

If you specifically want “researcher” (female), German often uses:

die Forscherin = female researcher.

But in many academic and scientific contexts, Wissenschaftlerin and Forscherin can overlap in meaning.